[v3,1/2] mm: vmalloc: Remove a global vmap_blocks xarray

  A global vmap_blocks-xarray array can be contented under
heavy usage of the vm_map_ram()/vm_unmap_ram() APIs. The
lock_stat shows that a "vmap_blocks.xa_lock" lock is a
second in a top-list when it comes to contentions:

<snip>
----------------------------------------
class name con-bounces contentions ...
----------------------------------------
vmap_area_lock:         2554079 2554276 ...
  --------------
  vmap_area_lock        1297948  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
  vmap_area_lock        1256330  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
  vmap_area_lock              1  [<00000000c95c05a7>] find_vm_area+0x16/0x70
  --------------
  vmap_area_lock        1738590  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
  vmap_area_lock         815688  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
  vmap_area_lock              1  [<00000000c1d619d7>] __get_vm_area_node+0xd2/0x170

vmap_blocks.xa_lock:    862689  862698 ...
  -------------------
  vmap_blocks.xa_lock   378418    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
  vmap_blocks.xa_lock   484280    [<00000000caa2ef03>] xa_erase+0xe/0x30
  -------------------
  vmap_blocks.xa_lock   576226    [<00000000caa2ef03>] xa_erase+0xe/0x30
  vmap_blocks.xa_lock   286472    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
...
<snip>

that is a result of running vm_map_ram()/vm_unmap_ram() in
a loop. The test creates 64(on 64 CPUs system) threads and
each one maps/unmaps 1 page.

After this change the "xa_lock" can be considered as a noise
in the same test condition:

<snip>
...
&xa->xa_lock#1:         10333 10394 ...
  --------------
  &xa->xa_lock#1        5349      [<00000000bbbc9751>] xa_erase+0xe/0x30
  &xa->xa_lock#1        5045      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
  --------------
  &xa->xa_lock#1        7326      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
  &xa->xa_lock#1        3068      [<00000000bbbc9751>] xa_erase+0xe/0x30
...
<snip>

This patch does not fix vmap_area_lock/free_vmap_area_lock and
purge_vmap_area_lock bottle-necks, it is rather a separate rework.

v1 - v2:
   - Add more comments(Andrew Morton req.)
   - Switch to WARN_ON_ONCE(Lorenzo Stoakes req.)

v2 -> v3:
   - Fix a kernel-doc complain(Matthew Wilcox)

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 mm/vmalloc.c | 85 +++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 64 insertions(+), 21 deletions(-)
  

On Mon, Mar 27, 2023 at 07:01:25PM +0200, Uladzislau Rezki (Sony) wrote:
> A global vmap_blocks-xarray array can be contented under
> heavy usage of the vm_map_ram()/vm_unmap_ram() APIs. The
> lock_stat shows that a "vmap_blocks.xa_lock" lock is a
> second in a top-list when it comes to contentions:
>
> <snip>
> ----------------------------------------
> class name con-bounces contentions ...
> ----------------------------------------
> vmap_area_lock:         2554079 2554276 ...
>   --------------
>   vmap_area_lock        1297948  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
>   vmap_area_lock        1256330  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
>   vmap_area_lock              1  [<00000000c95c05a7>] find_vm_area+0x16/0x70
>   --------------
>   vmap_area_lock        1738590  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
>   vmap_area_lock         815688  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
>   vmap_area_lock              1  [<00000000c1d619d7>] __get_vm_area_node+0xd2/0x170
>
> vmap_blocks.xa_lock:    862689  862698 ...
>   -------------------
>   vmap_blocks.xa_lock   378418    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
>   vmap_blocks.xa_lock   484280    [<00000000caa2ef03>] xa_erase+0xe/0x30
>   -------------------
>   vmap_blocks.xa_lock   576226    [<00000000caa2ef03>] xa_erase+0xe/0x30
>   vmap_blocks.xa_lock   286472    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
> ...
> <snip>
>
> that is a result of running vm_map_ram()/vm_unmap_ram() in
> a loop. The test creates 64(on 64 CPUs system) threads and
> each one maps/unmaps 1 page.
>
> After this change the "xa_lock" can be considered as a noise
> in the same test condition:
>
> <snip>
> ...
> &xa->xa_lock#1:         10333 10394 ...
>   --------------
>   &xa->xa_lock#1        5349      [<00000000bbbc9751>] xa_erase+0xe/0x30
>   &xa->xa_lock#1        5045      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
>   --------------
>   &xa->xa_lock#1        7326      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
>   &xa->xa_lock#1        3068      [<00000000bbbc9751>] xa_erase+0xe/0x30
> ...
> <snip>
>
> This patch does not fix vmap_area_lock/free_vmap_area_lock and
> purge_vmap_area_lock bottle-necks, it is rather a separate rework.
>
> v1 - v2:
>    - Add more comments(Andrew Morton req.)
>    - Switch to WARN_ON_ONCE(Lorenzo Stoakes req.)
>
> v2 -> v3:
>    - Fix a kernel-doc complain(Matthew Wilcox)
>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  mm/vmalloc.c | 85 +++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 64 insertions(+), 21 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 978194dc2bb8..821256ecf81c 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1908,9 +1908,22 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
>  #define VMAP_BLOCK		0x2 /* mark out the vmap_block sub-type*/
>  #define VMAP_FLAGS_MASK		0x3
>
> +/*
> + * We should probably have a fallback mechanism to allocate virtual memory
> + * out of partially filled vmap blocks. However vmap block sizing should be
> + * fairly reasonable according to the vmalloc size, so it shouldn't be a
> + * big problem.
> + */
>  struct vmap_block_queue {
>  	spinlock_t lock;
>  	struct list_head free;
> +
> +	/*
> +	 * An xarray requires an extra memory dynamically to
> +	 * be allocated. If it is an issue, we can use rb-tree
> +	 * instead.
> +	 */
> +	struct xarray vmap_blocks;
>  };
>
>  struct vmap_block {
> @@ -1928,24 +1941,46 @@ struct vmap_block {
>  static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
>
>  /*
> - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> - * in the free path. Could get rid of this if we change the API to return a
> - * "cookie" from alloc, to be passed to free. But no big deal yet.
> + * In order to fast access to any "vmap_block" associated with a
> + * specific address, we store them into a per-cpu xarray. A hash
> + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> + * value.
> + *
> + * Please note, a vmap_block_queue, which is a per-cpu, is not
> + * serialized by a raw_smp_processor_id() current CPU, instead
> + * it is chosen based on a CPU-index it belongs to, i.e. it is
> + * a hash-table.
> + *
> + * An example:
> + *
> + *  CPU_1  CPU_2  CPU_0
> + *    |      |      |
> + *    V      V      V
> + * 0     10     20     30     40     50     60
> + * |------|------|------|------|------|------|...<vmap address space>
> + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> + *
> + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> + *
> + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> + *
> + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
>   */

OK so if I understand this correctly, you're overloading the per-CPU
vmap_block_queue array to use as a simple hash based on the address and
relying on the xa_lock() in xa_insert() to serialise in case of contention?

I like the general heft of your comment but I feel this could be spelled
out a little more clearly, something like:-

  In order to have fast access to any vmap_block object associated with a
  specific address, we use a hash.

  Rather than waste space on defining a new hash table  we take advantage
  of the fact we already have a static per-cpu array vmap_block_queue.

  This is already used for per-CPU access to the block queue, however we
  overload this to _also_ act as a vmap_block hash. The hash function is
  addr_to_vbq() which hashes on vb->va->va_start.

  This then uses per_cpu() to lookup the _index_ rather than the
  _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are
  indexed on the same key as the hash (vb->va->va_start).

  xarray read acceses are protected by RCU lock and inserts are protected
  by a spin lock so there is no risk of a race here.

  An example:

  ...

Feel free to cut this down as needed :) but I do feel it's important to
_explicitly_ point out that we're overloading this as it's quite confusing
at face value.

> -static DEFINE_XARRAY(vmap_blocks);
> +static struct vmap_block_queue *
> +addr_to_vbq(unsigned long addr)
> +{
> +	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
>
> -/*
> - * We should probably have a fallback mechanism to allocate virtual memory
> - * out of partially filled vmap blocks. However vmap block sizing should be
> - * fairly reasonable according to the vmalloc size, so it shouldn't be a
> - * big problem.
> - */
> +	return &per_cpu(vmap_block_queue, index);
> +}
>
> -static unsigned long addr_to_vb_idx(unsigned long addr)
> +static unsigned long
> +addr_to_vb_va_start(unsigned long addr)
>  {
> -	addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
> -	addr /= VMAP_BLOCK_SIZE;
> -	return addr;
> +	return rounddown(addr, VMAP_BLOCK_SIZE);
>  }
>
>  static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
> @@ -1953,7 +1988,7 @@ static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
>  	unsigned long addr;
>
>  	addr = va_start + (pages_off << PAGE_SHIFT);
> -	BUG_ON(addr_to_vb_idx(addr) != addr_to_vb_idx(va_start));
> +	WARN_ON_ONCE(addr_to_vb_va_start(addr) != va_start);
>  	return (void *)addr;
>  }
>
> @@ -1970,7 +2005,6 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  	struct vmap_block_queue *vbq;
>  	struct vmap_block *vb;
>  	struct vmap_area *va;
> -	unsigned long vb_idx;
>  	int node, err;
>  	void *vaddr;
>
> @@ -2003,8 +2037,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  	bitmap_set(vb->used_map, 0, (1UL << order));
>  	INIT_LIST_HEAD(&vb->free_list);
>
> -	vb_idx = addr_to_vb_idx(va->va_start);
> -	err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> +	vbq = addr_to_vbq(va->va_start);
> +	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);

I might be being pedantic here, but shortly after this code you reassign vbq:-

	vbq = addr_to_vbq(va->va_start);
	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);
	if (err) {
		kfree(vb);
		free_vmap_area(va);
		return ERR_PTR(err);
	}

	vbq = raw_cpu_ptr(&vmap_block_queue);

Which is confusing at a glance, as you're using it once as a hash lookup
and again for its 'true purpose'.

I wonder whether it would be better overall, since you always follow a vbq
lookup explicitly with an operation on vmap_blocks, to just add a helper
that returned a pointer to the xarray? e.g. (untested code here :):-

static struct xarray *get_vblock_array(unsigned long addr)
{
	struct vmap_block_queue *vbq;
	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();

	vbq = &per_cpu(vmap_block_queue, index);
	return &vbq->vblocks;
}

And replace addr_to_vbq() with this. That'd also make the mechanism of this
hash lookup super explicit.

>  	if (err) {
>  		kfree(vb);
>  		free_vmap_area(va);
> @@ -2021,9 +2055,11 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>
>  static void free_vmap_block(struct vmap_block *vb)
>  {
> +	struct vmap_block_queue *vbq;
>  	struct vmap_block *tmp;
>
> -	tmp = xa_erase(&vmap_blocks, addr_to_vb_idx(vb->va->va_start));
> +	vbq = addr_to_vbq(vb->va->va_start);
> +	tmp = xa_erase(&vbq->vmap_blocks, vb->va->va_start);
>  	BUG_ON(tmp != vb);
>
>  	spin_lock(&vmap_area_lock);
> @@ -2135,6 +2171,7 @@ static void vb_free(unsigned long addr, unsigned long size)
>  	unsigned long offset;
>  	unsigned int order;
>  	struct vmap_block *vb;
> +	struct vmap_block_queue *vbq;
>
>  	BUG_ON(offset_in_page(size));
>  	BUG_ON(size > PAGE_SIZE*VMAP_MAX_ALLOC);
> @@ -2143,7 +2180,10 @@ static void vb_free(unsigned long addr, unsigned long size)
>
>  	order = get_order(size);
>  	offset = (addr & (VMAP_BLOCK_SIZE - 1)) >> PAGE_SHIFT;
> -	vb = xa_load(&vmap_blocks, addr_to_vb_idx(addr));
> +
> +	vbq = addr_to_vbq(addr);
> +	vb = xa_load(&vbq->vmap_blocks, addr_to_vb_va_start(addr));
> +
>  	spin_lock(&vb->lock);
>  	bitmap_clear(vb->used_map, offset, (1UL << order));
>  	spin_unlock(&vb->lock);
> @@ -3486,6 +3526,7 @@ static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags
>  {
>  	char *start;
>  	struct vmap_block *vb;
> +	struct vmap_block_queue *vbq;
>  	unsigned long offset;
>  	unsigned int rs, re, n;
>
> @@ -3503,7 +3544,8 @@ static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags
>  	 * Area is split into regions and tracked with vmap_block, read out
>  	 * each region and zero fill the hole between regions.
>  	 */
> -	vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long)addr));
> +	vbq = addr_to_vbq((unsigned long) addr);
> +	vb = xa_load(&vbq->vmap_blocks, addr_to_vb_va_start((unsigned long) addr));
>  	if (!vb)
>  		goto finished;
>
> @@ -4272,6 +4314,7 @@ void __init vmalloc_init(void)
>  		p = &per_cpu(vfree_deferred, i);
>  		init_llist_head(&p->list);
>  		INIT_WORK(&p->wq, delayed_vfree_work);
> +		xa_init(&vbq->vmap_blocks);
>  	}
>
>  	/* Import existing vmlist entries. */
> --
> 2.30.2
>
  

On 03/27/23 at 07:01pm, Uladzislau Rezki (Sony) wrote:
> A global vmap_blocks-xarray array can be contented under
> heavy usage of the vm_map_ram()/vm_unmap_ram() APIs. The
> lock_stat shows that a "vmap_blocks.xa_lock" lock is a
> second in a top-list when it comes to contentions:
> 
> <snip>
> ----------------------------------------
> class name con-bounces contentions ...
> ----------------------------------------
> vmap_area_lock:         2554079 2554276 ...
>   --------------
>   vmap_area_lock        1297948  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
>   vmap_area_lock        1256330  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
>   vmap_area_lock              1  [<00000000c95c05a7>] find_vm_area+0x16/0x70
>   --------------
>   vmap_area_lock        1738590  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
>   vmap_area_lock         815688  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
>   vmap_area_lock              1  [<00000000c1d619d7>] __get_vm_area_node+0xd2/0x170
> 
> vmap_blocks.xa_lock:    862689  862698 ...
>   -------------------
>   vmap_blocks.xa_lock   378418    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
>   vmap_blocks.xa_lock   484280    [<00000000caa2ef03>] xa_erase+0xe/0x30
>   -------------------
>   vmap_blocks.xa_lock   576226    [<00000000caa2ef03>] xa_erase+0xe/0x30
>   vmap_blocks.xa_lock   286472    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
> ...
> <snip>
> 
> that is a result of running vm_map_ram()/vm_unmap_ram() in
> a loop. The test creates 64(on 64 CPUs system) threads and
> each one maps/unmaps 1 page.

With my understanding, the xarray will take more time when calling
xa_insert() or xa_erase() because these two will cause xa_expand() and
xa_shrink() if the index is sparse. xa_load() should be low cost to
finish. Wondering if in your testing code, the mapping address is close
or too far.

1 mm/vmalloc.c <<new_vmap_block>>
  err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
2 mm/vmalloc.c <<free_vmap_block>>
  tmp = xa_erase(&vmap_blocks, addr_to_vb_idx(vb->va->va_start));
3 mm/vmalloc.c <<vb_free>>
  vb = xa_load(&vmap_blocks, addr_to_vb_idx(addr));
4 mm/vmalloc.c <<vmap_ram_vread_iter>>
  vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long )addr));

> 
> After this change the "xa_lock" can be considered as a noise
> in the same test condition:
> 
> <snip>
> ...
> &xa->xa_lock#1:         10333 10394 ...
>   --------------
>   &xa->xa_lock#1        5349      [<00000000bbbc9751>] xa_erase+0xe/0x30
>   &xa->xa_lock#1        5045      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
>   --------------
>   &xa->xa_lock#1        7326      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
>   &xa->xa_lock#1        3068      [<00000000bbbc9751>] xa_erase+0xe/0x30
> ...
> <snip>
> 
> This patch does not fix vmap_area_lock/free_vmap_area_lock and
> purge_vmap_area_lock bottle-necks, it is rather a separate rework.
> 
> v1 - v2:
>    - Add more comments(Andrew Morton req.)
>    - Switch to WARN_ON_ONCE(Lorenzo Stoakes req.)
> 
> v2 -> v3:
>    - Fix a kernel-doc complain(Matthew Wilcox)
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  mm/vmalloc.c | 85 +++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 64 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 978194dc2bb8..821256ecf81c 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1908,9 +1908,22 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
>  #define VMAP_BLOCK		0x2 /* mark out the vmap_block sub-type*/
>  #define VMAP_FLAGS_MASK		0x3
>  
> +/*
> + * We should probably have a fallback mechanism to allocate virtual memory
> + * out of partially filled vmap blocks. However vmap block sizing should be
> + * fairly reasonable according to the vmalloc size, so it shouldn't be a
> + * big problem.
> + */
>  struct vmap_block_queue {
>  	spinlock_t lock;
>  	struct list_head free;
> +
> +	/*
> +	 * An xarray requires an extra memory dynamically to
> +	 * be allocated. If it is an issue, we can use rb-tree
> +	 * instead.
> +	 */
> +	struct xarray vmap_blocks;
>  };
>  
>  struct vmap_block {
> @@ -1928,24 +1941,46 @@ struct vmap_block {
>  static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
>  
>  /*
> - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> - * in the free path. Could get rid of this if we change the API to return a
> - * "cookie" from alloc, to be passed to free. But no big deal yet.
> + * In order to fast access to any "vmap_block" associated with a
> + * specific address, we store them into a per-cpu xarray. A hash
> + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> + * value.
> + *
> + * Please note, a vmap_block_queue, which is a per-cpu, is not
> + * serialized by a raw_smp_processor_id() current CPU, instead
> + * it is chosen based on a CPU-index it belongs to, i.e. it is
> + * a hash-table.
> + *
> + * An example:
> + *
> + *  CPU_1  CPU_2  CPU_0
> + *    |      |      |
> + *    V      V      V
> + * 0     10     20     30     40     50     60
> + * |------|------|------|------|------|------|...<vmap address space>
> + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> + *
> + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> + *
> + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> + *
> + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
>   */
> -static DEFINE_XARRAY(vmap_blocks);
> +static struct vmap_block_queue *
> +addr_to_vbq(unsigned long addr)
> +{
> +	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
>  
> -/*
> - * We should probably have a fallback mechanism to allocate virtual memory
> - * out of partially filled vmap blocks. However vmap block sizing should be
> - * fairly reasonable according to the vmalloc size, so it shouldn't be a
> - * big problem.
> - */
> +	return &per_cpu(vmap_block_queue, index);
> +}
>  
> -static unsigned long addr_to_vb_idx(unsigned long addr)
> +static unsigned long
> +addr_to_vb_va_start(unsigned long addr)
>  {
> -	addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
> -	addr /= VMAP_BLOCK_SIZE;
> -	return addr;
> +	return rounddown(addr, VMAP_BLOCK_SIZE);
>  }
>  
>  static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
> @@ -1953,7 +1988,7 @@ static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
>  	unsigned long addr;
>  
>  	addr = va_start + (pages_off << PAGE_SHIFT);
> -	BUG_ON(addr_to_vb_idx(addr) != addr_to_vb_idx(va_start));
> +	WARN_ON_ONCE(addr_to_vb_va_start(addr) != va_start);
>  	return (void *)addr;
>  }
>  
> @@ -1970,7 +2005,6 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  	struct vmap_block_queue *vbq;
>  	struct vmap_block *vb;
>  	struct vmap_area *va;
> -	unsigned long vb_idx;
>  	int node, err;
>  	void *vaddr;
>  
> @@ -2003,8 +2037,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  	bitmap_set(vb->used_map, 0, (1UL << order));
>  	INIT_LIST_HEAD(&vb->free_list);
>  
> -	vb_idx = addr_to_vb_idx(va->va_start);
> -	err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> +	vbq = addr_to_vbq(va->va_start);
> +	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);

Using va->va_start as index to access xarray may cost extra memory.
Imagine we got a virtual address at VMALLOC_START, its region is
[VMALLOC_START, VMALLOC_START+4095]. In the xarray, its sequence order
is 0. While with va->va_start, it's 0xffffc90000000000UL on x86_64 with
level4 paging mode. That means for the first page size vmalloc area,
storing it into xarray need about 10 levels of xa_node, just for the one
page size. With the old addr_to_vb_idx(), its index is 0. Only one level
height is needed. One xa_node is about 72bytes, it could take more time
and memory to access va->va_start. Not sure if my understanding is correct.

static unsigned long addr_to_vb_idx(unsigned long addr)
{
        addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
        addr /= VMAP_BLOCK_SIZE;
        return addr;
}

>  	if (err) {
>  		kfree(vb);
>  		free_vmap_area(va);
> @@ -2021,9 +2055,11 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  
>  static void free_vmap_block(struct vmap_block *vb)
>  {
> +	struct vmap_block_queue *vbq;
>  	struct vmap_block *tmp;
>  
> -	tmp = xa_erase(&vmap_blocks, addr_to_vb_idx(vb->va->va_start));
> +	vbq = addr_to_vbq(vb->va->va_start);
> +	tmp = xa_erase(&vbq->vmap_blocks, vb->va->va_start);
>  	BUG_ON(tmp != vb);
>  
>  	spin_lock(&vmap_area_lock);
> @@ -2135,6 +2171,7 @@ static void vb_free(unsigned long addr, unsigned long size)
>  	unsigned long offset;
>  	unsigned int order;
>  	struct vmap_block *vb;
> +	struct vmap_block_queue *vbq;
>  
>  	BUG_ON(offset_in_page(size));
>  	BUG_ON(size > PAGE_SIZE*VMAP_MAX_ALLOC);
> @@ -2143,7 +2180,10 @@ static void vb_free(unsigned long addr, unsigned long size)
>  
>  	order = get_order(size);
>  	offset = (addr & (VMAP_BLOCK_SIZE - 1)) >> PAGE_SHIFT;
> -	vb = xa_load(&vmap_blocks, addr_to_vb_idx(addr));
> +
> +	vbq = addr_to_vbq(addr);
> +	vb = xa_load(&vbq->vmap_blocks, addr_to_vb_va_start(addr));
> +
>  	spin_lock(&vb->lock);
>  	bitmap_clear(vb->used_map, offset, (1UL << order));
>  	spin_unlock(&vb->lock);
> @@ -3486,6 +3526,7 @@ static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags
>  {
>  	char *start;
>  	struct vmap_block *vb;
> +	struct vmap_block_queue *vbq;
>  	unsigned long offset;
>  	unsigned int rs, re, n;
>  
> @@ -3503,7 +3544,8 @@ static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags
>  	 * Area is split into regions and tracked with vmap_block, read out
>  	 * each region and zero fill the hole between regions.
>  	 */
> -	vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long)addr));
> +	vbq = addr_to_vbq((unsigned long) addr);
> +	vb = xa_load(&vbq->vmap_blocks, addr_to_vb_va_start((unsigned long) addr));
>  	if (!vb)
>  		goto finished;
>  
> @@ -4272,6 +4314,7 @@ void __init vmalloc_init(void)
>  		p = &per_cpu(vfree_deferred, i);
>  		init_llist_head(&p->list);
>  		INIT_WORK(&p->wq, delayed_vfree_work);
> +		xa_init(&vbq->vmap_blocks);
>  	}
>  
>  	/* Import existing vmlist entries. */
> -- 
> 2.30.2
>
  

On Tue, Mar 28, 2023 at 11:25:54AM +0800, Baoquan He wrote:
> On 03/27/23 at 07:01pm, Uladzislau Rezki (Sony) wrote:
> > A global vmap_blocks-xarray array can be contented under
> > heavy usage of the vm_map_ram()/vm_unmap_ram() APIs. The
> > lock_stat shows that a "vmap_blocks.xa_lock" lock is a
> > second in a top-list when it comes to contentions:
> > 
> > <snip>
> > ----------------------------------------
> > class name con-bounces contentions ...
> > ----------------------------------------
> > vmap_area_lock:         2554079 2554276 ...
> >   --------------
> >   vmap_area_lock        1297948  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
> >   vmap_area_lock        1256330  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
> >   vmap_area_lock              1  [<00000000c95c05a7>] find_vm_area+0x16/0x70
> >   --------------
> >   vmap_area_lock        1738590  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
> >   vmap_area_lock         815688  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
> >   vmap_area_lock              1  [<00000000c1d619d7>] __get_vm_area_node+0xd2/0x170
> > 
> > vmap_blocks.xa_lock:    862689  862698 ...
> >   -------------------
> >   vmap_blocks.xa_lock   378418    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
> >   vmap_blocks.xa_lock   484280    [<00000000caa2ef03>] xa_erase+0xe/0x30
> >   -------------------
> >   vmap_blocks.xa_lock   576226    [<00000000caa2ef03>] xa_erase+0xe/0x30
> >   vmap_blocks.xa_lock   286472    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
> > ...
> > <snip>
> > 
> > that is a result of running vm_map_ram()/vm_unmap_ram() in
> > a loop. The test creates 64(on 64 CPUs system) threads and
> > each one maps/unmaps 1 page.
> 
> With my understanding, the xarray will take more time when calling
> xa_insert() or xa_erase() because these two will cause xa_expand() and
> xa_shrink() if the index is sparse. xa_load() should be low cost to
> finish. Wondering if in your testing code, the mapping address is close
> or too far.
> 
> 1 mm/vmalloc.c <<new_vmap_block>>
>   err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> 2 mm/vmalloc.c <<free_vmap_block>>
>   tmp = xa_erase(&vmap_blocks, addr_to_vb_idx(vb->va->va_start));
> 3 mm/vmalloc.c <<vb_free>>
>   vb = xa_load(&vmap_blocks, addr_to_vb_idx(addr));
> 4 mm/vmalloc.c <<vmap_ram_vread_iter>>
>   vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long )addr));
> 
> > 
> > After this change the "xa_lock" can be considered as a noise
> > in the same test condition:
> > 
> > <snip>
> > ...
> > &xa->xa_lock#1:         10333 10394 ...
> >   --------------
> >   &xa->xa_lock#1        5349      [<00000000bbbc9751>] xa_erase+0xe/0x30
> >   &xa->xa_lock#1        5045      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
> >   --------------
> >   &xa->xa_lock#1        7326      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
> >   &xa->xa_lock#1        3068      [<00000000bbbc9751>] xa_erase+0xe/0x30
> > ...
> > <snip>
> > 
> > This patch does not fix vmap_area_lock/free_vmap_area_lock and
> > purge_vmap_area_lock bottle-necks, it is rather a separate rework.
> > 
> > v1 - v2:
> >    - Add more comments(Andrew Morton req.)
> >    - Switch to WARN_ON_ONCE(Lorenzo Stoakes req.)
> > 
> > v2 -> v3:
> >    - Fix a kernel-doc complain(Matthew Wilcox)
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  mm/vmalloc.c | 85 +++++++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 64 insertions(+), 21 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 978194dc2bb8..821256ecf81c 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1908,9 +1908,22 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> >  #define VMAP_BLOCK		0x2 /* mark out the vmap_block sub-type*/
> >  #define VMAP_FLAGS_MASK		0x3
> >  
> > +/*
> > + * We should probably have a fallback mechanism to allocate virtual memory
> > + * out of partially filled vmap blocks. However vmap block sizing should be
> > + * fairly reasonable according to the vmalloc size, so it shouldn't be a
> > + * big problem.
> > + */
> >  struct vmap_block_queue {
> >  	spinlock_t lock;
> >  	struct list_head free;
> > +
> > +	/*
> > +	 * An xarray requires an extra memory dynamically to
> > +	 * be allocated. If it is an issue, we can use rb-tree
> > +	 * instead.
> > +	 */
> > +	struct xarray vmap_blocks;
> >  };
> >  
> >  struct vmap_block {
> > @@ -1928,24 +1941,46 @@ struct vmap_block {
> >  static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
> >  
> >  /*
> > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> > - * in the free path. Could get rid of this if we change the API to return a
> > - * "cookie" from alloc, to be passed to free. But no big deal yet.
> > + * In order to fast access to any "vmap_block" associated with a
> > + * specific address, we store them into a per-cpu xarray. A hash
> > + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> > + * value.
> > + *
> > + * Please note, a vmap_block_queue, which is a per-cpu, is not
> > + * serialized by a raw_smp_processor_id() current CPU, instead
> > + * it is chosen based on a CPU-index it belongs to, i.e. it is
> > + * a hash-table.
> > + *
> > + * An example:
> > + *
> > + *  CPU_1  CPU_2  CPU_0
> > + *    |      |      |
> > + *    V      V      V
> > + * 0     10     20     30     40     50     60
> > + * |------|------|------|------|------|------|...<vmap address space>
> > + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> > + *
> > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> > + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> > + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> > + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> >   */
> > -static DEFINE_XARRAY(vmap_blocks);
> > +static struct vmap_block_queue *
> > +addr_to_vbq(unsigned long addr)
> > +{
> > +	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> >  
> > -/*
> > - * We should probably have a fallback mechanism to allocate virtual memory
> > - * out of partially filled vmap blocks. However vmap block sizing should be
> > - * fairly reasonable according to the vmalloc size, so it shouldn't be a
> > - * big problem.
> > - */
> > +	return &per_cpu(vmap_block_queue, index);
> > +}
> >  
> > -static unsigned long addr_to_vb_idx(unsigned long addr)
> > +static unsigned long
> > +addr_to_vb_va_start(unsigned long addr)
> >  {
> > -	addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
> > -	addr /= VMAP_BLOCK_SIZE;
> > -	return addr;
> > +	return rounddown(addr, VMAP_BLOCK_SIZE);
> >  }
> >  
> >  static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
> > @@ -1953,7 +1988,7 @@ static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
> >  	unsigned long addr;
> >  
> >  	addr = va_start + (pages_off << PAGE_SHIFT);
> > -	BUG_ON(addr_to_vb_idx(addr) != addr_to_vb_idx(va_start));
> > +	WARN_ON_ONCE(addr_to_vb_va_start(addr) != va_start);
> >  	return (void *)addr;
> >  }
> >  
> > @@ -1970,7 +2005,6 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> >  	struct vmap_block_queue *vbq;
> >  	struct vmap_block *vb;
> >  	struct vmap_area *va;
> > -	unsigned long vb_idx;
> >  	int node, err;
> >  	void *vaddr;
> >  
> > @@ -2003,8 +2037,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> >  	bitmap_set(vb->used_map, 0, (1UL << order));
> >  	INIT_LIST_HEAD(&vb->free_list);
> >  
> > -	vb_idx = addr_to_vb_idx(va->va_start);
> > -	err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> > +	vbq = addr_to_vbq(va->va_start);
> > +	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);
> 
> Using va->va_start as index to access xarray may cost extra memory.
> Imagine we got a virtual address at VMALLOC_START, its region is
> [VMALLOC_START, VMALLOC_START+4095]. In the xarray, its sequence order
> is 0. While with va->va_start, it's 0xffffc90000000000UL on x86_64 with
> level4 paging mode. That means for the first page size vmalloc area,
> storing it into xarray need about 10 levels of xa_node, just for the one
> page size. With the old addr_to_vb_idx(), its index is 0. Only one level
> height is needed. One xa_node is about 72bytes, it could take more time
> and memory to access va->va_start. Not sure if my understanding is correct.
> 
> static unsigned long addr_to_vb_idx(unsigned long addr)
> {
>         addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
>         addr /= VMAP_BLOCK_SIZE;
>         return addr;
> }
> 
If the size of array depends on index "length", then, indeed it will require
more memory. From the other hand we can keep the old addr_to_vb_idx() function 
in order to "cut" a va->va_start index.

--
Uladzislau Rezki
  

On Mon, Mar 27, 2023 at 09:09:32PM +0100, Lorenzo Stoakes wrote:
> On Mon, Mar 27, 2023 at 07:01:25PM +0200, Uladzislau Rezki (Sony) wrote:
> > A global vmap_blocks-xarray array can be contented under
> > heavy usage of the vm_map_ram()/vm_unmap_ram() APIs. The
> > lock_stat shows that a "vmap_blocks.xa_lock" lock is a
> > second in a top-list when it comes to contentions:
> >
> > <snip>
> > ----------------------------------------
> > class name con-bounces contentions ...
> > ----------------------------------------
> > vmap_area_lock:         2554079 2554276 ...
> >   --------------
> >   vmap_area_lock        1297948  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
> >   vmap_area_lock        1256330  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
> >   vmap_area_lock              1  [<00000000c95c05a7>] find_vm_area+0x16/0x70
> >   --------------
> >   vmap_area_lock        1738590  [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
> >   vmap_area_lock         815688  [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
> >   vmap_area_lock              1  [<00000000c1d619d7>] __get_vm_area_node+0xd2/0x170
> >
> > vmap_blocks.xa_lock:    862689  862698 ...
> >   -------------------
> >   vmap_blocks.xa_lock   378418    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
> >   vmap_blocks.xa_lock   484280    [<00000000caa2ef03>] xa_erase+0xe/0x30
> >   -------------------
> >   vmap_blocks.xa_lock   576226    [<00000000caa2ef03>] xa_erase+0xe/0x30
> >   vmap_blocks.xa_lock   286472    [<00000000625a5626>] vm_map_ram+0x359/0x4a0
> > ...
> > <snip>
> >
> > that is a result of running vm_map_ram()/vm_unmap_ram() in
> > a loop. The test creates 64(on 64 CPUs system) threads and
> > each one maps/unmaps 1 page.
> >
> > After this change the "xa_lock" can be considered as a noise
> > in the same test condition:
> >
> > <snip>
> > ...
> > &xa->xa_lock#1:         10333 10394 ...
> >   --------------
> >   &xa->xa_lock#1        5349      [<00000000bbbc9751>] xa_erase+0xe/0x30
> >   &xa->xa_lock#1        5045      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
> >   --------------
> >   &xa->xa_lock#1        7326      [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
> >   &xa->xa_lock#1        3068      [<00000000bbbc9751>] xa_erase+0xe/0x30
> > ...
> > <snip>
> >
> > This patch does not fix vmap_area_lock/free_vmap_area_lock and
> > purge_vmap_area_lock bottle-necks, it is rather a separate rework.
> >
> > v1 - v2:
> >    - Add more comments(Andrew Morton req.)
> >    - Switch to WARN_ON_ONCE(Lorenzo Stoakes req.)
> >
> > v2 -> v3:
> >    - Fix a kernel-doc complain(Matthew Wilcox)
> >
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  mm/vmalloc.c | 85 +++++++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 64 insertions(+), 21 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 978194dc2bb8..821256ecf81c 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1908,9 +1908,22 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> >  #define VMAP_BLOCK		0x2 /* mark out the vmap_block sub-type*/
> >  #define VMAP_FLAGS_MASK		0x3
> >
> > +/*
> > + * We should probably have a fallback mechanism to allocate virtual memory
> > + * out of partially filled vmap blocks. However vmap block sizing should be
> > + * fairly reasonable according to the vmalloc size, so it shouldn't be a
> > + * big problem.
> > + */
> >  struct vmap_block_queue {
> >  	spinlock_t lock;
> >  	struct list_head free;
> > +
> > +	/*
> > +	 * An xarray requires an extra memory dynamically to
> > +	 * be allocated. If it is an issue, we can use rb-tree
> > +	 * instead.
> > +	 */
> > +	struct xarray vmap_blocks;
> >  };
> >
> >  struct vmap_block {
> > @@ -1928,24 +1941,46 @@ struct vmap_block {
> >  static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
> >
> >  /*
> > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> > - * in the free path. Could get rid of this if we change the API to return a
> > - * "cookie" from alloc, to be passed to free. But no big deal yet.
> > + * In order to fast access to any "vmap_block" associated with a
> > + * specific address, we store them into a per-cpu xarray. A hash
> > + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> > + * value.
> > + *
> > + * Please note, a vmap_block_queue, which is a per-cpu, is not
> > + * serialized by a raw_smp_processor_id() current CPU, instead
> > + * it is chosen based on a CPU-index it belongs to, i.e. it is
> > + * a hash-table.
> > + *
> > + * An example:
> > + *
> > + *  CPU_1  CPU_2  CPU_0
> > + *    |      |      |
> > + *    V      V      V
> > + * 0     10     20     30     40     50     60
> > + * |------|------|------|------|------|------|...<vmap address space>
> > + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> > + *
> > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> > + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> > + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> > + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> >   */
> 
> OK so if I understand this correctly, you're overloading the per-CPU
> vmap_block_queue array to use as a simple hash based on the address and
> relying on the xa_lock() in xa_insert() to serialise in case of contention?
> 
> I like the general heft of your comment but I feel this could be spelled
> out a little more clearly, something like:-
> 
>   In order to have fast access to any vmap_block object associated with a
>   specific address, we use a hash.
> 
>   Rather than waste space on defining a new hash table  we take advantage
>   of the fact we already have a static per-cpu array vmap_block_queue.
> 
>   This is already used for per-CPU access to the block queue, however we
>   overload this to _also_ act as a vmap_block hash. The hash function is
>   addr_to_vbq() which hashes on vb->va->va_start.
> 
>   This then uses per_cpu() to lookup the _index_ rather than the
>   _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are
>   indexed on the same key as the hash (vb->va->va_start).
> 
>   xarray read acceses are protected by RCU lock and inserts are protected
>   by a spin lock so there is no risk of a race here.
> 
>   An example:
> 
>   ...
> 
> Feel free to cut this down as needed :) but I do feel it's important to
> _explicitly_ point out that we're overloading this as it's quite confusing
> at face value.
> 
> > -static DEFINE_XARRAY(vmap_blocks);
> > +static struct vmap_block_queue *
> > +addr_to_vbq(unsigned long addr)
> > +{
> > +	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> >
> > -/*
> > - * We should probably have a fallback mechanism to allocate virtual memory
> > - * out of partially filled vmap blocks. However vmap block sizing should be
> > - * fairly reasonable according to the vmalloc size, so it shouldn't be a
> > - * big problem.
> > - */
> > +	return &per_cpu(vmap_block_queue, index);
> > +}
> >
> > -static unsigned long addr_to_vb_idx(unsigned long addr)
> > +static unsigned long
> > +addr_to_vb_va_start(unsigned long addr)
> >  {
> > -	addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
> > -	addr /= VMAP_BLOCK_SIZE;
> > -	return addr;
> > +	return rounddown(addr, VMAP_BLOCK_SIZE);
> >  }
> >
> >  static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
> > @@ -1953,7 +1988,7 @@ static void *vmap_block_vaddr(unsigned long va_start, unsigned long pages_off)
> >  	unsigned long addr;
> >
> >  	addr = va_start + (pages_off << PAGE_SHIFT);
> > -	BUG_ON(addr_to_vb_idx(addr) != addr_to_vb_idx(va_start));
> > +	WARN_ON_ONCE(addr_to_vb_va_start(addr) != va_start);
> >  	return (void *)addr;
> >  }
> >
> > @@ -1970,7 +2005,6 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> >  	struct vmap_block_queue *vbq;
> >  	struct vmap_block *vb;
> >  	struct vmap_area *va;
> > -	unsigned long vb_idx;
> >  	int node, err;
> >  	void *vaddr;
> >
> > @@ -2003,8 +2037,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> >  	bitmap_set(vb->used_map, 0, (1UL << order));
> >  	INIT_LIST_HEAD(&vb->free_list);
> >
> > -	vb_idx = addr_to_vb_idx(va->va_start);
> > -	err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> > +	vbq = addr_to_vbq(va->va_start);
> > +	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);
> 
> I might be being pedantic here, but shortly after this code you reassign vbq:-
> 
> 	vbq = addr_to_vbq(va->va_start);
> 	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);
> 	if (err) {
> 		kfree(vb);
> 		free_vmap_area(va);
> 		return ERR_PTR(err);
> 	}
> 
> 	vbq = raw_cpu_ptr(&vmap_block_queue);
> 
> Which is confusing at a glance, as you're using it once as a hash lookup
> and again for its 'true purpose'.
> 
> I wonder whether it would be better overall, since you always follow a vbq
> lookup explicitly with an operation on vmap_blocks, to just add a helper
> that returned a pointer to the xarray? e.g. (untested code here :):-
> 
> static struct xarray *get_vblock_array(unsigned long addr)
> {
> 	struct vmap_block_queue *vbq;
> 	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> 
> 	vbq = &per_cpu(vmap_block_queue, index);
> 	return &vbq->vblocks;
> }
> 
> And replace addr_to_vbq() with this. That'd also make the mechanism of this
> hash lookup super explicit.
> 
Thank you for the comments. I will go through all of them and fix
accordingly. At lease i see that i have to update the documentation in
more better way!

Thanks!

--
Uladzislau Rezki
  

> >  /*
> > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> > - * in the free path. Could get rid of this if we change the API to return a
> > - * "cookie" from alloc, to be passed to free. But no big deal yet.
> > + * In order to fast access to any "vmap_block" associated with a
> > + * specific address, we store them into a per-cpu xarray. A hash
> > + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> > + * value.
> > + *
> > + * Please note, a vmap_block_queue, which is a per-cpu, is not
> > + * serialized by a raw_smp_processor_id() current CPU, instead
> > + * it is chosen based on a CPU-index it belongs to, i.e. it is
> > + * a hash-table.
> > + *
> > + * An example:
> > + *
> > + *  CPU_1  CPU_2  CPU_0
> > + *    |      |      |
> > + *    V      V      V
> > + * 0     10     20     30     40     50     60
> > + * |------|------|------|------|------|------|...<vmap address space>
> > + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> > + *
> > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> > + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> > + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> > + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> >   */
> 
> OK so if I understand this correctly, you're overloading the per-CPU
> vmap_block_queue array to use as a simple hash based on the address and
> relying on the xa_lock() in xa_insert() to serialise in case of contention?
> 
Sorry i missed your question. You correctly understood what i am doing.

Basically, we can associate any address with an index in per-cpu-array.
Since a CPU pre-allocates a fixed block size, which is a VMAP_BLOCK_SIZE, 
we can map any address within this block to a certain index or i call
it a specific CPU zone it belongs to.

If we want fully serialize it we have to allocate a new vmap block in
CPU owner zone. According to ASCII picture, for CPU0 it is 0-20, 30-40
addresses. In fact, even though it would be "fully" serialized, in practise
id does not give a visible performance. So this is not needed and it
has extra drawbacks.

--
Uladzislau Rezki
  

On 03/28/23 at 02:34pm, Uladzislau Rezki wrote:
......  
> > > @@ -2003,8 +2037,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> > >  	bitmap_set(vb->used_map, 0, (1UL << order));
> > >  	INIT_LIST_HEAD(&vb->free_list);
> > >  
> > > -	vb_idx = addr_to_vb_idx(va->va_start);
> > > -	err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> > > +	vbq = addr_to_vbq(va->va_start);
> > > +	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);
> > 
> > Using va->va_start as index to access xarray may cost extra memory.
> > Imagine we got a virtual address at VMALLOC_START, its region is
> > [VMALLOC_START, VMALLOC_START+4095]. In the xarray, its sequence order
> > is 0. While with va->va_start, it's 0xffffc90000000000UL on x86_64 with
> > level4 paging mode. That means for the first page size vmalloc area,
> > storing it into xarray need about 10 levels of xa_node, just for the one
> > page size. With the old addr_to_vb_idx(), its index is 0. Only one level
> > height is needed. One xa_node is about 72bytes, it could take more time
> > and memory to access va->va_start. Not sure if my understanding is correct.
> > 
> > static unsigned long addr_to_vb_idx(unsigned long addr)
> > {
> >         addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
> >         addr /= VMAP_BLOCK_SIZE;
> >         return addr;
> > }
> > 
> If the size of array depends on index "length", then, indeed it will require
> more memory. From the other hand we can keep the old addr_to_vb_idx() function 
> in order to "cut" a va->va_start index.

Yeah, the extra 10 levels of xa_node is unnecessary if we keep the old
addr_to_vb_idx(). And the prolonged path will cost more time to reach the 
wanted leaf node. E.g on x86_64 with 4 level paging mode, vmalloc area
is 32TB. With the old calculation, its index range is [0, 8M], 4 level
heights of xa_node at most is enough to cover.
  

On Wed, Mar 29, 2023 at 12:33:05PM +0800, Baoquan He wrote:
> On 03/28/23 at 02:34pm, Uladzislau Rezki wrote:
> ......  
> > > > @@ -2003,8 +2037,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> > > >  	bitmap_set(vb->used_map, 0, (1UL << order));
> > > >  	INIT_LIST_HEAD(&vb->free_list);
> > > >  
> > > > -	vb_idx = addr_to_vb_idx(va->va_start);
> > > > -	err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> > > > +	vbq = addr_to_vbq(va->va_start);
> > > > +	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);
> > > 
> > > Using va->va_start as index to access xarray may cost extra memory.
> > > Imagine we got a virtual address at VMALLOC_START, its region is
> > > [VMALLOC_START, VMALLOC_START+4095]. In the xarray, its sequence order
> > > is 0. While with va->va_start, it's 0xffffc90000000000UL on x86_64 with
> > > level4 paging mode. That means for the first page size vmalloc area,
> > > storing it into xarray need about 10 levels of xa_node, just for the one
> > > page size. With the old addr_to_vb_idx(), its index is 0. Only one level
> > > height is needed. One xa_node is about 72bytes, it could take more time
> > > and memory to access va->va_start. Not sure if my understanding is correct.
> > > 
> > > static unsigned long addr_to_vb_idx(unsigned long addr)
> > > {
> > >         addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
> > >         addr /= VMAP_BLOCK_SIZE;
> > >         return addr;
> > > }
> > > 
> > If the size of array depends on index "length", then, indeed it will require
> > more memory. From the other hand we can keep the old addr_to_vb_idx() function 
> > in order to "cut" a va->va_start index.
> 
> Yeah, the extra 10 levels of xa_node is unnecessary if we keep the old
> addr_to_vb_idx(). And the prolonged path will cost more time to reach the 
> wanted leaf node. E.g on x86_64 with 4 level paging mode, vmalloc area
> is 32TB. With the old calculation, its index range is [0, 8M], 4 level
> heights of xa_node at most is enough to cover.
> 
Good! I have not analyzed how xarray stores its indexes. I will update
the patch to cut indexes so we stay the same as we used to be before.

--
Uladzislau Rezki
  

Hello, Lorenzo!

> >  /*
> > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> > - * in the free path. Could get rid of this if we change the API to return a
> > - * "cookie" from alloc, to be passed to free. But no big deal yet.
> > + * In order to fast access to any "vmap_block" associated with a
> > + * specific address, we store them into a per-cpu xarray. A hash
> > + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> > + * value.
> > + *
> > + * Please note, a vmap_block_queue, which is a per-cpu, is not
> > + * serialized by a raw_smp_processor_id() current CPU, instead
> > + * it is chosen based on a CPU-index it belongs to, i.e. it is
> > + * a hash-table.
> > + *
> > + * An example:
> > + *
> > + *  CPU_1  CPU_2  CPU_0
> > + *    |      |      |
> > + *    V      V      V
> > + * 0     10     20     30     40     50     60
> > + * |------|------|------|------|------|------|...<vmap address space>
> > + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> > + *
> > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> > + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> > + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> > + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> >   */
> 
> OK so if I understand this correctly, you're overloading the per-CPU
> vmap_block_queue array to use as a simple hash based on the address and
> relying on the xa_lock() in xa_insert() to serialise in case of contention?
> 
> I like the general heft of your comment but I feel this could be spelled
> out a little more clearly, something like:-
> 
>   In order to have fast access to any vmap_block object associated with a
>   specific address, we use a hash.
> 
>   Rather than waste space on defining a new hash table  we take advantage
>   of the fact we already have a static per-cpu array vmap_block_queue.
> 
>   This is already used for per-CPU access to the block queue, however we
>   overload this to _also_ act as a vmap_block hash. The hash function is
>   addr_to_vbq() which hashes on vb->va->va_start.
> 
>   This then uses per_cpu() to lookup the _index_ rather than the
>   _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are
>   indexed on the same key as the hash (vb->va->va_start).
> 
>   xarray read acceses are protected by RCU lock and inserts are protected
>   by a spin lock so there is no risk of a race here.
> 
/*
 * In order to fast access to any "vmap_block" associated with a
 * specific address, we use a hash.
 *
 * A per-cpu vmap_block_queue is used in both ways, to serialize
 * an access to free block chains among CPUs(alloc path) and it
 * also acts as a vmap_block hash(alloc/free paths). It means we
 * overload it, since we already have the per-cpu array which is
 * used as a hash table.
 *
 * A hash function is addr_to_vbq() which hashes any address to
 * a specific index(in a hash) it belongs to. This then uses a
 * per_cpu() macro to access the array with specific index.
 *
 * An example:
 *
 *  CPU_1  CPU_2  CPU_0
 *    |      |      |
 *    V      V      V
 * 0     10     20     30     40     50     60
 * |------|------|------|------|------|------|...<vmap address space>
 *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
 *
 * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
 *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
 *
 * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
 *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
 *
 * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
 *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
 *
 * This technique allows almost remove a lock-contention in locking
 * primitives which protect insert/remove operations.
 */
Are you find with it?

--
Uladzislau Rezki
  

On Wed, Mar 29, 2023 at 05:01:11PM +0200, Uladzislau Rezki wrote:
> Hello, Lorenzo!
>
> > >  /*
> > > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> > > - * in the free path. Could get rid of this if we change the API to return a
> > > - * "cookie" from alloc, to be passed to free. But no big deal yet.
> > > + * In order to fast access to any "vmap_block" associated with a
> > > + * specific address, we store them into a per-cpu xarray. A hash
> > > + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> > > + * value.
> > > + *
> > > + * Please note, a vmap_block_queue, which is a per-cpu, is not
> > > + * serialized by a raw_smp_processor_id() current CPU, instead
> > > + * it is chosen based on a CPU-index it belongs to, i.e. it is
> > > + * a hash-table.
> > > + *
> > > + * An example:
> > > + *
> > > + *  CPU_1  CPU_2  CPU_0
> > > + *    |      |      |
> > > + *    V      V      V
> > > + * 0     10     20     30     40     50     60
> > > + * |------|------|------|------|------|------|...<vmap address space>
> > > + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> > > + *
> > > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> > > + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> > > + *
> > > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> > > + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> > > + *
> > > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> > > + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> > >   */
> >
> > OK so if I understand this correctly, you're overloading the per-CPU
> > vmap_block_queue array to use as a simple hash based on the address and
> > relying on the xa_lock() in xa_insert() to serialise in case of contention?
> >
> > I like the general heft of your comment but I feel this could be spelled
> > out a little more clearly, something like:-
> >
> >   In order to have fast access to any vmap_block object associated with a
> >   specific address, we use a hash.
> >
> >   Rather than waste space on defining a new hash table  we take advantage
> >   of the fact we already have a static per-cpu array vmap_block_queue.
> >
> >   This is already used for per-CPU access to the block queue, however we
> >   overload this to _also_ act as a vmap_block hash. The hash function is
> >   addr_to_vbq() which hashes on vb->va->va_start.
> >
> >   This then uses per_cpu() to lookup the _index_ rather than the
> >   _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are
> >   indexed on the same key as the hash (vb->va->va_start).
> >
> >   xarray read acceses are protected by RCU lock and inserts are protected
> >   by a spin lock so there is no risk of a race here.
> >
> /*
>  * In order to fast access to any "vmap_block" associated with a
>  * specific address, we use a hash.
>  *
>  * A per-cpu vmap_block_queue is used in both ways, to serialize
>  * an access to free block chains among CPUs(alloc path) and it
>  * also acts as a vmap_block hash(alloc/free paths). It means we
>  * overload it, since we already have the per-cpu array which is
>  * used as a hash table.

Nit - it may be worth highlighting that when used as a hash it the 'cpu' is
not in fact a cpu but rather a hash key.

E.g. just add on the end of this something like:-

When used as a hash table the 'cpu' passed to per_cpu is not actually a CPU
but rather the hash key.

>  *
>  * A hash function is addr_to_vbq() which hashes any address to
>  * a specific index(in a hash) it belongs to. This then uses a
>  * per_cpu() macro to access the array with specific index.

May need a tweak if you are happy with my review that we can simply have a
helper that returns the xarray in which case we won't necessary have this
function :) but depends of course on how the respin looks!

>  *
>  * An example:
>  *
>  *  CPU_1  CPU_2  CPU_0
>  *    |      |      |
>  *    V      V      V
>  * 0     10     20     30     40     50     60
>  * |------|------|------|------|------|------|...<vmap address space>
>  *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
>  *
>  * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
>  *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
>  *
>  * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
>  *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
>  *
>  * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
>  *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
>  *
>  * This technique allows almost remove a lock-contention in locking
>  * primitives which protect insert/remove operations.

This sentence is a little confusing, perhaps rephrase a little:-

This technique almost always avoids lock contention on insert/remove,
however the xarray spinlock protects against any contention that remains.

>  */
> Are you find with it?

Other than the small nits above (sorry!) it seems fine! Thanks for
updating, much appreciated :)

>
> --
> Uladzislau Rezki
>
  

On Wed, Mar 29, 2023 at 05:23:04PM +0100, Lorenzo Stoakes wrote:
> On Wed, Mar 29, 2023 at 05:01:11PM +0200, Uladzislau Rezki wrote:
> > Hello, Lorenzo!
> >
> > > >  /*
> > > > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> > > > - * in the free path. Could get rid of this if we change the API to return a
> > > > - * "cookie" from alloc, to be passed to free. But no big deal yet.
> > > > + * In order to fast access to any "vmap_block" associated with a
> > > > + * specific address, we store them into a per-cpu xarray. A hash
> > > > + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> > > > + * value.
> > > > + *
> > > > + * Please note, a vmap_block_queue, which is a per-cpu, is not
> > > > + * serialized by a raw_smp_processor_id() current CPU, instead
> > > > + * it is chosen based on a CPU-index it belongs to, i.e. it is
> > > > + * a hash-table.
> > > > + *
> > > > + * An example:
> > > > + *
> > > > + *  CPU_1  CPU_2  CPU_0
> > > > + *    |      |      |
> > > > + *    V      V      V
> > > > + * 0     10     20     30     40     50     60
> > > > + * |------|------|------|------|------|------|...<vmap address space>
> > > > + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> > > > + *
> > > > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> > > > + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> > > > + *
> > > > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> > > > + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> > > > + *
> > > > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> > > > + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> > > >   */
> > >
> > > OK so if I understand this correctly, you're overloading the per-CPU
> > > vmap_block_queue array to use as a simple hash based on the address and
> > > relying on the xa_lock() in xa_insert() to serialise in case of contention?
> > >
> > > I like the general heft of your comment but I feel this could be spelled
> > > out a little more clearly, something like:-
> > >
> > >   In order to have fast access to any vmap_block object associated with a
> > >   specific address, we use a hash.
> > >
> > >   Rather than waste space on defining a new hash table  we take advantage
> > >   of the fact we already have a static per-cpu array vmap_block_queue.
> > >
> > >   This is already used for per-CPU access to the block queue, however we
> > >   overload this to _also_ act as a vmap_block hash. The hash function is
> > >   addr_to_vbq() which hashes on vb->va->va_start.
> > >
> > >   This then uses per_cpu() to lookup the _index_ rather than the
> > >   _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are
> > >   indexed on the same key as the hash (vb->va->va_start).
> > >
> > >   xarray read acceses are protected by RCU lock and inserts are protected
> > >   by a spin lock so there is no risk of a race here.
> > >
> > /*
> >  * In order to fast access to any "vmap_block" associated with a
> >  * specific address, we use a hash.
> >  *
> >  * A per-cpu vmap_block_queue is used in both ways, to serialize
> >  * an access to free block chains among CPUs(alloc path) and it
> >  * also acts as a vmap_block hash(alloc/free paths). It means we
> >  * overload it, since we already have the per-cpu array which is
> >  * used as a hash table.
> 
> Nit - it may be worth highlighting that when used as a hash it the 'cpu' is
> not in fact a cpu but rather a hash key.
> 
> E.g. just add on the end of this something like:-
> 
> When used as a hash table the 'cpu' passed to per_cpu is not actually a CPU
> but rather the hash key.
> 
> >  *
> >  * A hash function is addr_to_vbq() which hashes any address to
> >  * a specific index(in a hash) it belongs to. This then uses a
> >  * per_cpu() macro to access the array with specific index.
> 
> May need a tweak if you are happy with my review that we can simply have a
> helper that returns the xarray in which case we won't necessary have this
> function :) but depends of course on how the respin looks!
> 
> >  *
> >  * An example:
> >  *
> >  *  CPU_1  CPU_2  CPU_0
> >  *    |      |      |
> >  *    V      V      V
> >  * 0     10     20     30     40     50     60
> >  * |------|------|------|------|------|------|...<vmap address space>
> >  *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> >  *
> >  * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> >  *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> >  *
> >  * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> >  *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> >  *
> >  * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> >  *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> >  *
> >  * This technique allows almost remove a lock-contention in locking
> >  * primitives which protect insert/remove operations.
> 
> This sentence is a little confusing, perhaps rephrase a little:-
> 
> This technique almost always avoids lock contention on insert/remove,
> however the xarray spinlock protects against any contention that remains.
> 
> >  */
> > Are you find with it?
> 
> Other than the small nits above (sorry!) it seems fine! Thanks for
> updating, much appreciated :)
> 
Good. Made the changes. I will upload a new vX patch. Everything
that makes it more clear for readers is worth to do :)

--
Uladzislau Rezki