[v4] mm/page_ext: Do not allocate space for page_ext->flags if not needed

  There is 8 byte page_ext->flags field allocated per page whenever
CONFIG_PAGE_EXTENSION is enabled. However, not every user of page_ext
uses flags. Therefore, check whether flags is needed at least by one
user and if so allocate space for it.

For example when page_table_check is enabled, on a machine with 128G
of memory before the fix:

[    2.244288] allocated 536870912 bytes of page_ext
after the fix:
[    2.160154] allocated 268435456 bytes of page_ext

Also, add a kernel-doc comment before page_ext_operations that describes
the fields, and remove check if need() is set, as that is now a required
field.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
---
 include/linux/page_ext.h | 18 ++++++++++++++++++
 mm/page_ext.c            | 14 ++++++++++++--
 mm/page_owner.c          |  1 +
 mm/page_table_check.c    |  1 +
 4 files changed, 32 insertions(+), 2 deletions(-)

Changelog:
v4: Addressed comments from Mike Rapoport, added Acked-by's.
v3: Added comment before page_ext_operations, removed check if
    need is null.
v2: Fixed field name in page_owner.c that caused build error.
  

On Tue, Jan 17, 2023 at 08:21:03PM +0000, Pasha Tatashin wrote:
> There is 8 byte page_ext->flags field allocated per page whenever
> CONFIG_PAGE_EXTENSION is enabled. However, not every user of page_ext
> uses flags. Therefore, check whether flags is needed at least by one
> user and if so allocate space for it.
> 
> For example when page_table_check is enabled, on a machine with 128G
> of memory before the fix:
> 
> [    2.244288] allocated 536870912 bytes of page_ext
> after the fix:
> [    2.160154] allocated 268435456 bytes of page_ext
> 
> Also, add a kernel-doc comment before page_ext_operations that describes
> the fields, and remove check if need() is set, as that is now a required
> field.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

> ---
>  include/linux/page_ext.h | 18 ++++++++++++++++++
>  mm/page_ext.c            | 14 ++++++++++++--
>  mm/page_owner.c          |  1 +
>  mm/page_table_check.c    |  1 +
>  4 files changed, 32 insertions(+), 2 deletions(-)
> 
> Changelog:
> v4: Addressed comments from Mike Rapoport, added Acked-by's.
> v3: Added comment before page_ext_operations, removed check if
>     need is null.
> v2: Fixed field name in page_owner.c that caused build error.
> 
> diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
> index 22be4582faae..67314f648aeb 100644
> --- a/include/linux/page_ext.h
> +++ b/include/linux/page_ext.h
> @@ -7,15 +7,33 @@
>  #include <linux/stackdepot.h>
>  
>  struct pglist_data;
> +
> +/**
> + * struct page_ext_operations - per page_ext client operations
> + * @offset: Offset to the client's data within page_ext. Offset is returned to
> + *          the client by page_ext_init.
> + * @size: The size of the client data within page_ext.
> + * @need: Function that returns true if client requires page_ext.
> + * @init: (optional) Called to initialize client once page_exts are allocated.
> + * @need_shared_flags: True when client is using shared page_ext->flags
> + *                     field.
> + *
> + * Each Page Extension client must define page_ext_operations in
> + * page_ext_ops array.
> + */
>  struct page_ext_operations {
>  	size_t offset;
>  	size_t size;
>  	bool (*need)(void);
>  	void (*init)(void);
> +	bool need_shared_flags;
>  };
>  
>  #ifdef CONFIG_PAGE_EXTENSION
>  
> +/*
> + * The page_ext_flags users must set need_shared_flags to true.
> + */
>  enum page_ext_flags {
>  	PAGE_EXT_OWNER,
>  	PAGE_EXT_OWNER_ALLOCATED,
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 4ee522fd381c..e2c22ffdbb81 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -71,6 +71,7 @@ static bool need_page_idle(void)
>  }
>  static struct page_ext_operations page_idle_ops __initdata = {
>  	.need = need_page_idle,
> +	.need_shared_flags = true,
>  };
>  #endif
>  
> @@ -86,7 +87,7 @@ static struct page_ext_operations *page_ext_ops[] __initdata = {
>  #endif
>  };
>  
> -unsigned long page_ext_size = sizeof(struct page_ext);
> +unsigned long page_ext_size;
>  
>  static unsigned long total_usage;
>  static struct page_ext *lookup_page_ext(const struct page *page);
> @@ -106,7 +107,16 @@ static bool __init invoke_need_callbacks(void)
>  	bool need = false;
>  
>  	for (i = 0; i < entries; i++) {
> -		if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
> +		if (page_ext_ops[i]->need()) {
> +			if (page_ext_ops[i]->need_shared_flags) {
> +				page_ext_size = sizeof(struct page_ext);
> +				break;
> +			}
> +		}
> +	}
> +
> +	for (i = 0; i < entries; i++) {
> +		if (page_ext_ops[i]->need()) {
>  			page_ext_ops[i]->offset = page_ext_size;
>  			page_ext_size += page_ext_ops[i]->size;
>  			need = true;
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 2d27f532df4c..f0553bedb39d 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -99,6 +99,7 @@ struct page_ext_operations page_owner_ops = {
>  	.size = sizeof(struct page_owner),
>  	.need = need_page_owner,
>  	.init = init_page_owner,
> +	.need_shared_flags = true,
>  };
>  
>  static inline struct page_owner *get_page_owner(struct page_ext *page_ext)
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index 93e633c1d587..25d8610c0042 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -45,6 +45,7 @@ struct page_ext_operations page_table_check_ops = {
>  	.size = sizeof(struct page_table_check),
>  	.need = need_page_table_check,
>  	.init = init_page_table_check,
> +	.need_shared_flags = false,
>  };
>  
>  static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
> -- 
> 2.39.0.314.g84b9a713c41-goog
>
  

On Tue, 17 Jan 2023, Pasha Tatashin wrote:

> There is 8 byte page_ext->flags field allocated per page whenever
> CONFIG_PAGE_EXTENSION is enabled. However, not every user of page_ext
> uses flags. Therefore, check whether flags is needed at least by one
> user and if so allocate space for it.
> 
> For example when page_table_check is enabled, on a machine with 128G
> of memory before the fix:
> 
> [    2.244288] allocated 536870912 bytes of page_ext
> after the fix:
> [    2.160154] allocated 268435456 bytes of page_ext
> 
> Also, add a kernel-doc comment before page_ext_operations that describes
> the fields, and remove check if need() is set, as that is now a required
> field.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Hildenbrand <david@redhat.com>

Acked-by: David Rientjes <rientjes@google.com>