[v2,12/14] arm64/mm: Wire up PTE_CONT for user mappings

  With the ptep API sufficiently refactored, we can now introduce a new
"contpte" API layer, which transparently manages the PTE_CONT bit for
user mappings. Whenever it detects a set of PTEs that meet the
requirements for a contiguous range, the PTEs are re-painted with the
PTE_CONT bit. Use of contpte mappings is intended to be transparent to
the core-mm, which continues to interact with individual ptes.

Since a contpte block only has a single access and dirty bit, the
semantic here changes slightly; when getting a pte (e.g. ptep_get())
that is part of a contpte mapping, the access and dirty information are
pulled from the block (so all ptes in the block return the same
access/dirty info). When changing the access/dirty info on a pte (e.g.
ptep_set_access_flags()) that is part of a contpte mapping, this change
will affect the whole contpte block. This is works fine in practice
since we guarrantee that only a single folio is mapped by a contpte
block, and the core-mm tracks access/dirty information per folio.

This initial change provides a baseline that can be optimized in future
commits. That said, fold/unfold operations (which imply tlb
invalidation) are avoided where possible with a few tricks for
access/dirty bit management. Write-protect modifications for contpte
mappings are currently non-optimal, and incure a regression in fork()
performance. This will be addressed in follow-up changes.

In order for the public functions, which used to be pure inline, to
continue to be callable by modules, export all the contpte_* symbols
that are now called by those public inline functions.

The feature is enabled/disabled with the ARM64_CONTPTE Kconfig parameter
at build time. It defaults to enabled as long as its dependency,
TRANSPARENT_HUGEPAGE is also enabled. The core-mm depends upon
TRANSPARENT_HUGEPAGE to be able to allocate large folios, so if its not
enabled, then there is no chance of meeting the physical contiguity
requirement for contpte mappings.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 arch/arm64/Kconfig               |  10 +-
 arch/arm64/include/asm/pgtable.h | 202 ++++++++++++++++++
 arch/arm64/mm/Makefile           |   1 +
 arch/arm64/mm/contpte.c          | 351 +++++++++++++++++++++++++++++++
 4 files changed, 563 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm64/mm/contpte.c
  

Ryan Roberts <ryan.roberts@arm.com> writes:

[...]

> +static void contpte_fold(struct mm_struct *mm, unsigned long addr,
> +			pte_t *ptep, pte_t pte, bool fold)
> +{
> +	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
> +	unsigned long start_addr;
> +	pte_t *start_ptep;
> +	int i;
> +
> +	start_ptep = ptep = contpte_align_down(ptep);
> +	start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +	pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
> +	pte = fold ? pte_mkcont(pte) : pte_mknoncont(pte);
> +
> +	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
> +		pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
> +
> +		if (pte_dirty(ptent))
> +			pte = pte_mkdirty(pte);
> +
> +		if (pte_young(ptent))
> +			pte = pte_mkyoung(pte);
> +	}
> +
> +	__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
> +
> +	__set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
> +}
> +
> +void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
> +			pte_t *ptep, pte_t pte)
> +{
> +	/*
> +	 * We have already checked that the virtual and pysical addresses are
> +	 * correctly aligned for a contpte mapping in contpte_try_fold() so the
> +	 * remaining checks are to ensure that the contpte range is fully
> +	 * covered by a single folio, and ensure that all the ptes are valid
> +	 * with contiguous PFNs and matching prots. We ignore the state of the
> +	 * access and dirty bits for the purpose of deciding if its a contiguous
> +	 * range; the folding process will generate a single contpte entry which
> +	 * has a single access and dirty bit. Those 2 bits are the logical OR of
> +	 * their respective bits in the constituent pte entries. In order to
> +	 * ensure the contpte range is covered by a single folio, we must
> +	 * recover the folio from the pfn, but special mappings don't have a
> +	 * folio backing them. Fortunately contpte_try_fold() already checked
> +	 * that the pte is not special - we never try to fold special mappings.
> +	 * Note we can't use vm_normal_page() for this since we don't have the
> +	 * vma.
> +	 */
> +
> +	struct page *page = pte_page(pte);
> +	struct folio *folio = page_folio(page);
> +	unsigned long folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
> +	unsigned long folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
> +	unsigned long cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +	unsigned long cont_eaddr = cont_saddr + CONT_PTE_SIZE;
> +	unsigned long pfn;
> +	pgprot_t prot;
> +	pte_t subpte;
> +	pte_t *orig_ptep;
> +	int i;
> +
> +	if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
> +		return;
> +
> +	pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
> +	prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
> +	orig_ptep = ptep;
> +	ptep = contpte_align_down(ptep);
> +
> +	for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
> +		subpte = __ptep_get(ptep);
> +		subpte = pte_mkold(pte_mkclean(subpte));
> +
> +		if (!pte_valid(subpte) ||
> +		    pte_pfn(subpte) != pfn ||
> +		    pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot))
> +			return;
> +	}
> +
> +	contpte_fold(mm, addr, orig_ptep, pte, true);
> +}
> +EXPORT_SYMBOL(__contpte_try_fold);
> +
> +void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> +			pte_t *ptep, pte_t pte)
> +{
> +	/*
> +	 * We have already checked that the ptes are contiguous in
> +	 * contpte_try_unfold(), so we can unfold unconditionally here.
> +	 */
> +
> +	contpte_fold(mm, addr, ptep, pte, false);

I'm still working my way through the series but calling a fold during an
unfold stood out as it seemed wrong. Obviously further reading revealed
the boolean flag that changes the functions meaning but I think it would
be better to refactor that.

We could easily rename contpte_fold() to eg. set_cont_ptes() and factor
the pte calculation loop into a separate helper
(eg. calculate_contpte_dirty_young() or some hopefully better name)
called further up the stack. That has an added benefit of providing a
spot to add the nice comment for young/dirty rules you provided in the
patch description ;-)

In other words we'd have something like:

void __contpte_try_unfold() {
     pte = calculate_contpte_dirty_young(mm, addr, ptep, pte);
     pte = pte_mknoncont(pte);
     set_cont_ptes(mm, addr, ptep, pte);
}

Which IMHO is more immediately understandable.

 - Alistair

> +}
> +EXPORT_SYMBOL(__contpte_try_unfold);
> +
> +pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> +{
> +	/*
> +	 * Gather access/dirty bits, which may be populated in any of the ptes
> +	 * of the contig range. We are guarranteed to be holding the PTL, so any
> +	 * contiguous range cannot be unfolded or otherwise modified under our
> +	 * feet.
> +	 */
> +
> +	pte_t pte;
> +	int i;
> +
> +	ptep = contpte_align_down(ptep);
> +
> +	for (i = 0; i < CONT_PTES; i++, ptep++) {
> +		pte = __ptep_get(ptep);
> +
> +		if (pte_dirty(pte))
> +			orig_pte = pte_mkdirty(orig_pte);
> +
> +		if (pte_young(pte))
> +			orig_pte = pte_mkyoung(orig_pte);
> +	}
> +
> +	return orig_pte;
> +}
> +EXPORT_SYMBOL(contpte_ptep_get);
> +
> +pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> +{
> +	/*
> +	 * Gather access/dirty bits, which may be populated in any of the ptes
> +	 * of the contig range. We may not be holding the PTL, so any contiguous
> +	 * range may be unfolded/modified/refolded under our feet. Therefore we
> +	 * ensure we read a _consistent_ contpte range by checking that all ptes
> +	 * in the range are valid and have CONT_PTE set, that all pfns are
> +	 * contiguous and that all pgprots are the same (ignoring access/dirty).
> +	 * If we find a pte that is not consistent, then we must be racing with
> +	 * an update so start again. If the target pte does not have CONT_PTE
> +	 * set then that is considered consistent on its own because it is not
> +	 * part of a contpte range.
> +	 */
> +
> +	pte_t orig_pte;
> +	pgprot_t orig_prot;
> +	pte_t *ptep;
> +	unsigned long pfn;
> +	pte_t pte;
> +	pgprot_t prot;
> +	int i;
> +
> +retry:
> +	orig_pte = __ptep_get(orig_ptep);
> +
> +	if (!pte_valid_cont(orig_pte))
> +		return orig_pte;
> +
> +	orig_prot = pte_pgprot(pte_mkold(pte_mkclean(orig_pte)));
> +	ptep = contpte_align_down(orig_ptep);
> +	pfn = pte_pfn(orig_pte) - (orig_ptep - ptep);
> +
> +	for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
> +		pte = __ptep_get(ptep);
> +		prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
> +
> +		if (!pte_valid_cont(pte) ||
> +		   pte_pfn(pte) != pfn ||
> +		   pgprot_val(prot) != pgprot_val(orig_prot))
> +			goto retry;
> +
> +		if (pte_dirty(pte))
> +			orig_pte = pte_mkdirty(orig_pte);
> +
> +		if (pte_young(pte))
> +			orig_pte = pte_mkyoung(orig_pte);
> +	}
> +
> +	return orig_pte;
> +}
> +EXPORT_SYMBOL(contpte_ptep_get_lockless);
> +
> +void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> +					pte_t *ptep, pte_t pte, unsigned int nr)
> +{
> +	unsigned long next;
> +	unsigned long end = addr + (nr << PAGE_SHIFT);
> +	unsigned long pfn = pte_pfn(pte);
> +	pgprot_t prot = pte_pgprot(pte);
> +	pte_t orig_pte;
> +
> +	do {
> +		next = pte_cont_addr_end(addr, end);
> +		nr = (next - addr) >> PAGE_SHIFT;
> +		pte = pfn_pte(pfn, prot);
> +
> +		if (((addr | next | (pfn << PAGE_SHIFT)) & ~CONT_PTE_MASK) == 0)
> +			pte = pte_mkcont(pte);
> +		else
> +			pte = pte_mknoncont(pte);
> +
> +		/*
> +		 * If operating on a partial contiguous range then we must first
> +		 * unfold the contiguous range if it was previously folded.
> +		 * Otherwise we could end up with overlapping tlb entries.
> +		 */
> +		if (nr != CONT_PTES)
> +			contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> +
> +		/*
> +		 * If we are replacing ptes that were contiguous or if the new
> +		 * ptes are contiguous and any of the ptes being replaced are
> +		 * valid, we need to clear and flush the range to prevent
> +		 * overlapping tlb entries.
> +		 */
> +		orig_pte = __ptep_get(ptep);
> +		if (pte_valid_cont(orig_pte) ||
> +		    (pte_cont(pte) && ptep_any_valid(ptep, nr)))
> +			ptep_clear_flush_range(mm, addr, ptep, nr);
> +
> +		__set_ptes(mm, addr, ptep, pte, nr);
> +
> +		addr = next;
> +		ptep += nr;
> +		pfn += nr;
> +
> +	} while (addr != end);
> +}
> +EXPORT_SYMBOL(contpte_set_ptes);
> +
> +int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> +					unsigned long addr, pte_t *ptep)
> +{
> +	/*
> +	 * ptep_clear_flush_young() technically requires us to clear the access
> +	 * flag for a _single_ pte. However, the core-mm code actually tracks
> +	 * access/dirty per folio, not per page. And since we only create a
> +	 * contig range when the range is covered by a single folio, we can get
> +	 * away with clearing young for the whole contig range here, so we avoid
> +	 * having to unfold.
> +	 */
> +
> +	int i;
> +	int young = 0;
> +
> +	ptep = contpte_align_down(ptep);
> +	addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +
> +	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
> +		young |= __ptep_test_and_clear_young(vma, addr, ptep);
> +
> +	return young;
> +}
> +EXPORT_SYMBOL(contpte_ptep_test_and_clear_young);
> +
> +int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> +					unsigned long addr, pte_t *ptep)
> +{
> +	int young;
> +
> +	young = contpte_ptep_test_and_clear_young(vma, addr, ptep);
> +
> +	if (young) {
> +		/*
> +		 * See comment in __ptep_clear_flush_young(); same rationale for
> +		 * eliding the trailing DSB applies here.
> +		 */
> +		addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +		__flush_tlb_range_nosync(vma, addr, addr + CONT_PTE_SIZE,
> +					 PAGE_SIZE, true, 3);
> +	}
> +
> +	return young;
> +}
> +EXPORT_SYMBOL(contpte_ptep_clear_flush_young);
> +
> +int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
> +					unsigned long addr, pte_t *ptep,
> +					pte_t entry, int dirty)
> +{
> +	pte_t orig_pte;
> +	int i;
> +	unsigned long start_addr;
> +
> +	/*
> +	 * Gather the access/dirty bits for the contiguous range. If nothing has
> +	 * changed, its a noop.
> +	 */
> +	orig_pte = ptep_get(ptep);
> +	if (pte_val(orig_pte) == pte_val(entry))
> +		return 0;
> +
> +	/*
> +	 * We can fix up access/dirty bits without having to unfold/fold the
> +	 * contig range. But if the write bit is changing, we need to go through
> +	 * the full unfold/fold cycle.
> +	 */
> +	if (pte_write(orig_pte) == pte_write(entry)) {
> +		/*
> +		 * For HW access management, we technically only need to update
> +		 * the flag on a single pte in the range. But for SW access
> +		 * management, we need to update all the ptes to prevent extra
> +		 * faults. Avoid per-page tlb flush in __ptep_set_access_flags()
> +		 * and instead flush the whole range at the end.
> +		 */
> +		ptep = contpte_align_down(ptep);
> +		start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +
> +		for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
> +			__ptep_set_access_flags(vma, addr, ptep, entry, 0);
> +
> +		if (dirty)
> +			__flush_tlb_range(vma, start_addr, addr,
> +							PAGE_SIZE, true, 3);
> +	} else {
> +		__contpte_try_unfold(vma->vm_mm, addr, ptep, orig_pte);
> +		__ptep_set_access_flags(vma, addr, ptep, entry, dirty);
> +		contpte_try_fold(vma->vm_mm, addr, ptep, entry);
> +	}
> +
> +	return 1;
> +}
> +EXPORT_SYMBOL(contpte_ptep_set_access_flags);
  

On 21/11/2023 11:22, Alistair Popple wrote:
> 
> Ryan Roberts <ryan.roberts@arm.com> writes:
> 
> [...]
> 
>> +static void contpte_fold(struct mm_struct *mm, unsigned long addr,
>> +			pte_t *ptep, pte_t pte, bool fold)
>> +{
>> +	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
>> +	unsigned long start_addr;
>> +	pte_t *start_ptep;
>> +	int i;
>> +
>> +	start_ptep = ptep = contpte_align_down(ptep);
>> +	start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>> +	pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
>> +	pte = fold ? pte_mkcont(pte) : pte_mknoncont(pte);
>> +
>> +	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
>> +		pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
>> +
>> +		if (pte_dirty(ptent))
>> +			pte = pte_mkdirty(pte);
>> +
>> +		if (pte_young(ptent))
>> +			pte = pte_mkyoung(pte);
>> +	}
>> +
>> +	__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
>> +
>> +	__set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
>> +}
>> +
>> +void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
>> +			pte_t *ptep, pte_t pte)
>> +{
>> +	/*
>> +	 * We have already checked that the virtual and pysical addresses are
>> +	 * correctly aligned for a contpte mapping in contpte_try_fold() so the
>> +	 * remaining checks are to ensure that the contpte range is fully
>> +	 * covered by a single folio, and ensure that all the ptes are valid
>> +	 * with contiguous PFNs and matching prots. We ignore the state of the
>> +	 * access and dirty bits for the purpose of deciding if its a contiguous
>> +	 * range; the folding process will generate a single contpte entry which
>> +	 * has a single access and dirty bit. Those 2 bits are the logical OR of
>> +	 * their respective bits in the constituent pte entries. In order to
>> +	 * ensure the contpte range is covered by a single folio, we must
>> +	 * recover the folio from the pfn, but special mappings don't have a
>> +	 * folio backing them. Fortunately contpte_try_fold() already checked
>> +	 * that the pte is not special - we never try to fold special mappings.
>> +	 * Note we can't use vm_normal_page() for this since we don't have the
>> +	 * vma.
>> +	 */
>> +
>> +	struct page *page = pte_page(pte);
>> +	struct folio *folio = page_folio(page);
>> +	unsigned long folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
>> +	unsigned long folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
>> +	unsigned long cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>> +	unsigned long cont_eaddr = cont_saddr + CONT_PTE_SIZE;
>> +	unsigned long pfn;
>> +	pgprot_t prot;
>> +	pte_t subpte;
>> +	pte_t *orig_ptep;
>> +	int i;
>> +
>> +	if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
>> +		return;
>> +
>> +	pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
>> +	prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
>> +	orig_ptep = ptep;
>> +	ptep = contpte_align_down(ptep);
>> +
>> +	for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
>> +		subpte = __ptep_get(ptep);
>> +		subpte = pte_mkold(pte_mkclean(subpte));
>> +
>> +		if (!pte_valid(subpte) ||
>> +		    pte_pfn(subpte) != pfn ||
>> +		    pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot))
>> +			return;
>> +	}
>> +
>> +	contpte_fold(mm, addr, orig_ptep, pte, true);
>> +}
>> +EXPORT_SYMBOL(__contpte_try_fold);
>> +
>> +void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>> +			pte_t *ptep, pte_t pte)
>> +{
>> +	/*
>> +	 * We have already checked that the ptes are contiguous in
>> +	 * contpte_try_unfold(), so we can unfold unconditionally here.
>> +	 */
>> +
>> +	contpte_fold(mm, addr, ptep, pte, false);
> 
> I'm still working my way through the series but 

Thanks for taking the time to review!

> calling a fold during an
> unfold stood out as it seemed wrong. Obviously further reading revealed
> the boolean flag that changes the functions meaning but I think it would
> be better to refactor that.

Yes that sounds reasonable.

> 
> We could easily rename contpte_fold() to eg. set_cont_ptes() and factor
> the pte calculation loop into a separate helper
> (eg. calculate_contpte_dirty_young() or some hopefully better name)
> called further up the stack. That has an added benefit of providing a
> spot to add the nice comment for young/dirty rules you provided in the
> patch description ;-)
> 
> In other words we'd have something like:
> 
> void __contpte_try_unfold() {
>      pte = calculate_contpte_dirty_young(mm, addr, ptep, pte);
>      pte = pte_mknoncont(pte);
>      set_cont_ptes(mm, addr, ptep, pte);
> }

My concern with this approach is that calculate_contpte_dirty_young() has side
effects; it has to clear each PTE as it loops through it prevent a race between
our reading access/dirty and another thread causing access/dirty to be set. So
its not just a "calculation", its the teardown portion of the process too. I
guess its a taste thing, so happy for it to be argued the other way, but I would
prefer to keep it all together in one function.

How about renaming contpte_fold() to contpte_convert() or contpte_repaint()
(other suggestions welcome), and extracting the pte_mkcont()/pte_mknoncont()
part (so we can remove the bool param):

void __contpte_try_unfold() {
	pte = pte_mknoncont(pte);
	contpte_convert(mm, addr, ptep, pte);
}

Thanks,
Ryan

> 
> Which IMHO is more immediately understandable.
> 
>  - Alistair
>
  

Ryan Roberts <ryan.roberts@arm.com> writes:

> On 21/11/2023 11:22, Alistair Popple wrote:
>> 
>> Ryan Roberts <ryan.roberts@arm.com> writes:
>> 
>> [...]
>> 
>>> +static void contpte_fold(struct mm_struct *mm, unsigned long addr,
>>> +			pte_t *ptep, pte_t pte, bool fold)
>>> +{
>>> +	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
>>> +	unsigned long start_addr;
>>> +	pte_t *start_ptep;
>>> +	int i;
>>> +
>>> +	start_ptep = ptep = contpte_align_down(ptep);
>>> +	start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>> +	pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
>>> +	pte = fold ? pte_mkcont(pte) : pte_mknoncont(pte);
>>> +
>>> +	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
>>> +		pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
>>> +
>>> +		if (pte_dirty(ptent))
>>> +			pte = pte_mkdirty(pte);
>>> +
>>> +		if (pte_young(ptent))
>>> +			pte = pte_mkyoung(pte);
>>> +	}
>>> +
>>> +	__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
>>> +
>>> +	__set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
>>> +}
>>> +
>>> +void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
>>> +			pte_t *ptep, pte_t pte)
>>> +{
>>> +	/*
>>> +	 * We have already checked that the virtual and pysical addresses are
>>> +	 * correctly aligned for a contpte mapping in contpte_try_fold() so the
>>> +	 * remaining checks are to ensure that the contpte range is fully
>>> +	 * covered by a single folio, and ensure that all the ptes are valid
>>> +	 * with contiguous PFNs and matching prots. We ignore the state of the
>>> +	 * access and dirty bits for the purpose of deciding if its a contiguous
>>> +	 * range; the folding process will generate a single contpte entry which
>>> +	 * has a single access and dirty bit. Those 2 bits are the logical OR of
>>> +	 * their respective bits in the constituent pte entries. In order to
>>> +	 * ensure the contpte range is covered by a single folio, we must
>>> +	 * recover the folio from the pfn, but special mappings don't have a
>>> +	 * folio backing them. Fortunately contpte_try_fold() already checked
>>> +	 * that the pte is not special - we never try to fold special mappings.
>>> +	 * Note we can't use vm_normal_page() for this since we don't have the
>>> +	 * vma.
>>> +	 */
>>> +
>>> +	struct page *page = pte_page(pte);
>>> +	struct folio *folio = page_folio(page);
>>> +	unsigned long folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
>>> +	unsigned long folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
>>> +	unsigned long cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>> +	unsigned long cont_eaddr = cont_saddr + CONT_PTE_SIZE;
>>> +	unsigned long pfn;
>>> +	pgprot_t prot;
>>> +	pte_t subpte;
>>> +	pte_t *orig_ptep;
>>> +	int i;
>>> +
>>> +	if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
>>> +		return;
>>> +
>>> +	pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
>>> +	prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
>>> +	orig_ptep = ptep;
>>> +	ptep = contpte_align_down(ptep);
>>> +
>>> +	for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
>>> +		subpte = __ptep_get(ptep);
>>> +		subpte = pte_mkold(pte_mkclean(subpte));
>>> +
>>> +		if (!pte_valid(subpte) ||
>>> +		    pte_pfn(subpte) != pfn ||
>>> +		    pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot))
>>> +			return;
>>> +	}
>>> +
>>> +	contpte_fold(mm, addr, orig_ptep, pte, true);
>>> +}
>>> +EXPORT_SYMBOL(__contpte_try_fold);
>>> +
>>> +void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>>> +			pte_t *ptep, pte_t pte)
>>> +{
>>> +	/*
>>> +	 * We have already checked that the ptes are contiguous in
>>> +	 * contpte_try_unfold(), so we can unfold unconditionally here.
>>> +	 */
>>> +
>>> +	contpte_fold(mm, addr, ptep, pte, false);
>> 
>> I'm still working my way through the series but 
>
> Thanks for taking the time to review!
>
>> calling a fold during an
>> unfold stood out as it seemed wrong. Obviously further reading revealed
>> the boolean flag that changes the functions meaning but I think it would
>> be better to refactor that.
>
> Yes that sounds reasonable.
>
>> 
>> We could easily rename contpte_fold() to eg. set_cont_ptes() and factor
>> the pte calculation loop into a separate helper
>> (eg. calculate_contpte_dirty_young() or some hopefully better name)
>> called further up the stack. That has an added benefit of providing a
>> spot to add the nice comment for young/dirty rules you provided in the
>> patch description ;-)
>> 
>> In other words we'd have something like:
>> 
>> void __contpte_try_unfold() {
>>      pte = calculate_contpte_dirty_young(mm, addr, ptep, pte);
>>      pte = pte_mknoncont(pte);
>>      set_cont_ptes(mm, addr, ptep, pte);
>> }
>
> My concern with this approach is that calculate_contpte_dirty_young() has side
> effects; it has to clear each PTE as it loops through it prevent a race between
> our reading access/dirty and another thread causing access/dirty to be set. So
> its not just a "calculation", its the teardown portion of the process too. I
> guess its a taste thing, so happy for it to be argued the other way, but I would
> prefer to keep it all together in one function.
>
> How about renaming contpte_fold() to contpte_convert() or contpte_repaint()
> (other suggestions welcome), and extracting the pte_mkcont()/pte_mknoncont()
> part (so we can remove the bool param):
>
> void __contpte_try_unfold() {
> 	pte = pte_mknoncont(pte);
> 	contpte_convert(mm, addr, ptep, pte);
> }

Thanks. That works for me, although sadly I don't have any better ideas
for names atm.

 - Alistair

> Thanks,
> Ryan
>
>> 
>> Which IMHO is more immediately understandable.
>> 
>>  - Alistair
>>
  

On 22/11/2023 06:01, Alistair Popple wrote:
> 
> Ryan Roberts <ryan.roberts@arm.com> writes:
> 
>> On 21/11/2023 11:22, Alistair Popple wrote:
>>>
>>> Ryan Roberts <ryan.roberts@arm.com> writes:
>>>
>>> [...]
>>>
>>>> +static void contpte_fold(struct mm_struct *mm, unsigned long addr,
>>>> +			pte_t *ptep, pte_t pte, bool fold)
>>>> +{
>>>> +	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
>>>> +	unsigned long start_addr;
>>>> +	pte_t *start_ptep;
>>>> +	int i;
>>>> +
>>>> +	start_ptep = ptep = contpte_align_down(ptep);
>>>> +	start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>>> +	pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
>>>> +	pte = fold ? pte_mkcont(pte) : pte_mknoncont(pte);
>>>> +
>>>> +	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
>>>> +		pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
>>>> +
>>>> +		if (pte_dirty(ptent))
>>>> +			pte = pte_mkdirty(pte);
>>>> +
>>>> +		if (pte_young(ptent))
>>>> +			pte = pte_mkyoung(pte);
>>>> +	}
>>>> +
>>>> +	__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
>>>> +
>>>> +	__set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
>>>> +}
>>>> +
>>>> +void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
>>>> +			pte_t *ptep, pte_t pte)
>>>> +{
>>>> +	/*
>>>> +	 * We have already checked that the virtual and pysical addresses are
>>>> +	 * correctly aligned for a contpte mapping in contpte_try_fold() so the
>>>> +	 * remaining checks are to ensure that the contpte range is fully
>>>> +	 * covered by a single folio, and ensure that all the ptes are valid
>>>> +	 * with contiguous PFNs and matching prots. We ignore the state of the
>>>> +	 * access and dirty bits for the purpose of deciding if its a contiguous
>>>> +	 * range; the folding process will generate a single contpte entry which
>>>> +	 * has a single access and dirty bit. Those 2 bits are the logical OR of
>>>> +	 * their respective bits in the constituent pte entries. In order to
>>>> +	 * ensure the contpte range is covered by a single folio, we must
>>>> +	 * recover the folio from the pfn, but special mappings don't have a
>>>> +	 * folio backing them. Fortunately contpte_try_fold() already checked
>>>> +	 * that the pte is not special - we never try to fold special mappings.
>>>> +	 * Note we can't use vm_normal_page() for this since we don't have the
>>>> +	 * vma.
>>>> +	 */
>>>> +
>>>> +	struct page *page = pte_page(pte);
>>>> +	struct folio *folio = page_folio(page);
>>>> +	unsigned long folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
>>>> +	unsigned long folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
>>>> +	unsigned long cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>>> +	unsigned long cont_eaddr = cont_saddr + CONT_PTE_SIZE;
>>>> +	unsigned long pfn;
>>>> +	pgprot_t prot;
>>>> +	pte_t subpte;
>>>> +	pte_t *orig_ptep;
>>>> +	int i;
>>>> +
>>>> +	if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
>>>> +		return;
>>>> +
>>>> +	pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
>>>> +	prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
>>>> +	orig_ptep = ptep;
>>>> +	ptep = contpte_align_down(ptep);
>>>> +
>>>> +	for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
>>>> +		subpte = __ptep_get(ptep);
>>>> +		subpte = pte_mkold(pte_mkclean(subpte));
>>>> +
>>>> +		if (!pte_valid(subpte) ||
>>>> +		    pte_pfn(subpte) != pfn ||
>>>> +		    pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot))
>>>> +			return;
>>>> +	}
>>>> +
>>>> +	contpte_fold(mm, addr, orig_ptep, pte, true);
>>>> +}
>>>> +EXPORT_SYMBOL(__contpte_try_fold);
>>>> +
>>>> +void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>>>> +			pte_t *ptep, pte_t pte)
>>>> +{
>>>> +	/*
>>>> +	 * We have already checked that the ptes are contiguous in
>>>> +	 * contpte_try_unfold(), so we can unfold unconditionally here.
>>>> +	 */
>>>> +
>>>> +	contpte_fold(mm, addr, ptep, pte, false);
>>>
>>> I'm still working my way through the series but 
>>
>> Thanks for taking the time to review!
>>
>>> calling a fold during an
>>> unfold stood out as it seemed wrong. Obviously further reading revealed
>>> the boolean flag that changes the functions meaning but I think it would
>>> be better to refactor that.
>>
>> Yes that sounds reasonable.
>>
>>>
>>> We could easily rename contpte_fold() to eg. set_cont_ptes() and factor
>>> the pte calculation loop into a separate helper
>>> (eg. calculate_contpte_dirty_young() or some hopefully better name)
>>> called further up the stack. That has an added benefit of providing a
>>> spot to add the nice comment for young/dirty rules you provided in the
>>> patch description ;-)
>>>
>>> In other words we'd have something like:
>>>
>>> void __contpte_try_unfold() {
>>>      pte = calculate_contpte_dirty_young(mm, addr, ptep, pte);
>>>      pte = pte_mknoncont(pte);
>>>      set_cont_ptes(mm, addr, ptep, pte);
>>> }
>>
>> My concern with this approach is that calculate_contpte_dirty_young() has side
>> effects; it has to clear each PTE as it loops through it prevent a race between
>> our reading access/dirty and another thread causing access/dirty to be set. So
>> its not just a "calculation", its the teardown portion of the process too. I
>> guess its a taste thing, so happy for it to be argued the other way, but I would
>> prefer to keep it all together in one function.
>>
>> How about renaming contpte_fold() to contpte_convert() or contpte_repaint()
>> (other suggestions welcome), and extracting the pte_mkcont()/pte_mknoncont()
>> part (so we can remove the bool param):
>>
>> void __contpte_try_unfold() {
>> 	pte = pte_mknoncont(pte);
>> 	contpte_convert(mm, addr, ptep, pte);
>> }
> 
> Thanks. That works for me, although sadly I don't have any better ideas
> for names atm.

Thanks - I'll make this change for v3 and go with contpte_convert().

> 
>  - Alistair
> 
>> Thanks,
>> Ryan
>>
>>>
>>> Which IMHO is more immediately understandable.
>>>
>>>  - Alistair
>>>
>