Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings

Mark Rutland
2024-02-12T12:00:31Z

Hi Ryan,

Overall this looks pretty good; I have a bunch of minor comments below, and a
bigger question on the way ptep_get_lockless() works.

On Fri, Feb 02, 2024 at 08:07:50AM +0000, Ryan Roberts wrote:
> 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.
>
> In this initial implementation, only suitable batches of PTEs, set via
> set_ptes(), are mapped with the PTE_CONT bit. Any subsequent
> modification of individual PTEs will cause an "unfold" operation to
> repaint the contpte block as individual PTEs before performing the
> requested operation. While, a modification of a single PTE could cause
> the block of PTEs to which it belongs to become eligible for "folding"
> into a contpte entry, "folding" is not performed in this initial
> implementation due to the costs of checking the requirements are met.
> Due to this, contpte mappings will degrade back to normal pte mappings
> over time if/when protections are changed. This will be solved in a
> future patch.
>
> 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 guarantee that only a single folio is mapped by a contpte
> block, and the core-mm tracks access/dirty information per folio.
>
> 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.
>
> Tested-by: John Hubbard <jhubbard@nvidia.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> arch/arm64/Kconfig | 9 +
> arch/arm64/include/asm/pgtable.h | 161 ++++++++++++++++++
> arch/arm64/mm/Makefile | 1 +
> arch/arm64/mm/contpte.c | 283 +++++++++++++++++++++++++++++++
> 4 files changed, 454 insertions(+)
> create mode 100644 arch/arm64/mm/contpte.c
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index d86d7f4758b5..1442e8ed95b6 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2230,6 +2230,15 @@ config UNWIND_PATCH_PAC_INTO_SCS
> select UNWIND_TABLES
> select DYNAMIC_SCS
>
> +config ARM64_CONTPTE
> + bool "Contiguous PTE mappings for user memory" if EXPERT
> + depends on TRANSPARENT_HUGEPAGE
> + default y
> + help
> + When enabled, user mappings are configured using the PTE contiguous
> + bit, for any mappings that meet the size and alignment requirements.
> + This reduces TLB pressure and improves performance.
> +
> endmenu # "Kernel Features"
>
> menu "Boot options"
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 7dc6b68ee516..34892a95403d 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -133,6 +133,10 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
> */
> #define pte_valid_not_user(pte) \
> ((pte_val(pte) & (PTE_VALID | PTE_USER | PTE_UXN)) == (PTE_VALID | PTE_UXN))
> +/*
> + * Returns true if the pte is valid and has the contiguous bit set.
> + */
> +#define pte_valid_cont(pte) (pte_valid(pte) && pte_cont(pte))
> /*
> * Could the pte be present in the TLB? We must check mm_tlb_flush_pending
> * so that we don't erroneously return false for pages that have been
> @@ -1135,6 +1139,161 @@ void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte);
> #define vmemmap_update_pte vmemmap_update_pte
> #endif
>
> +#ifdef CONFIG_ARM64_CONTPTE
> +
> +/*
> + * The contpte APIs are used to transparently manage the contiguous bit in ptes
> + * where it is possible and makes sense to do so. The PTE_CONT bit is considered
> + * a private implementation detail of the public ptep API (see below).
> + */
> +extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte);
> +extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
> +extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
> +extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte, unsigned int nr);
> +extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep);
> +extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep);
> +extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep,
> + pte_t entry, int dirty);
> +
> +static inline void contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte)
> +{
> + if (unlikely(pte_valid_cont(pte)))
> + __contpte_try_unfold(mm, addr, ptep, pte);
> +}
> +
> +/*
> + * The below functions constitute the public API that arm64 presents to the
> + * core-mm to manipulate PTE entries within their page tables (or at least this
> + * is the subset of the API that arm64 needs to implement). These public
> + * versions will automatically and transparently apply the contiguous bit where
> + * it makes sense to do so. Therefore any users that are contig-aware (e.g.
> + * hugetlb, kernel mapper) should NOT use these APIs, but instead use the
> + * private versions, which are prefixed with double underscore. All of these
> + * APIs except for ptep_get_lockless() are expected to be called with the PTL
> + * held.
> + */
> +
> +#define ptep_get ptep_get
> +static inline pte_t ptep_get(pte_t *ptep)
> +{
> + pte_t pte = __ptep_get(ptep);
> +
> + if (likely(!pte_valid_cont(pte)))
> + return pte;
> +
> + return contpte_ptep_get(ptep, pte);
> +}
> +
> +#define ptep_get_lockless ptep_get_lockless
> +static inline pte_t ptep_get_lockless(pte_t *ptep)
> +{
> + pte_t pte = __ptep_get(ptep);
> +
> + if (likely(!pte_valid_cont(pte)))
> + return pte;
> +
> + return contpte_ptep_get_lockless(ptep);
> +}
> +
> +static inline void set_pte(pte_t *ptep, pte_t pte)
> +{
> + /*
> + * We don't have the mm or vaddr so cannot unfold contig entries (since
> + * it requires tlb maintenance). set_pte() is not used in core code, so
> + * this should never even be called. Regardless do our best to service
> + * any call and emit a warning if there is any attempt to set a pte on
> + * top of an existing contig range.
> + */
> + pte_t orig_pte = __ptep_get(ptep);
> +
> + WARN_ON_ONCE(pte_valid_cont(orig_pte));
> + __set_pte(ptep, pte_mknoncont(pte));
> +}
> +
> +#define set_ptes set_ptes
> +static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte, unsigned int nr)
> +{
> + pte = pte_mknoncont(pte);

Why do we have to clear the contiguous bit here? Is that for the same reason as
set_pte(), or do we expect callers to legitimately call this with the
contiguous bit set in 'pte'?

I think you explained this to me in-person, and IIRC we don't expect callers to
go set the bit themselves, but since it 'leaks' out to them via __ptep_get() we
have to clear it here to defer the decision of whether to set/clear it when
modifying entries. It would be nice if we could have a description of why/when
we need to clear this, e.g. in the 'public API' comment block above.

> +
> + if (likely(nr == 1)) {
> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> + __set_ptes(mm, addr, ptep, pte, 1);
> + } else {
> + contpte_set_ptes(mm, addr, ptep, pte, nr);
> + }
> +}
> +
> +static inline void pte_clear(struct mm_struct *mm,
> + unsigned long addr, pte_t *ptep)
> +{
> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> + __pte_clear(mm, addr, ptep);
> +}
> +
> +#define __HAVE_ARCH_PTEP_GET_AND_CLEAR
> +static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> + unsigned long addr, pte_t *ptep)
> +{
> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> + return __ptep_get_and_clear(mm, addr, ptep);
> +}
> +
> +#define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
> +static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep)
> +{
> + pte_t orig_pte = __ptep_get(ptep);
> +
> + if (likely(!pte_valid_cont(orig_pte)))
> + return __ptep_test_and_clear_young(vma, addr, ptep);
> +
> + return contpte_ptep_test_and_clear_young(vma, addr, ptep);
> +}
> +
> +#define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
> +static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep)
> +{
> + pte_t orig_pte = __ptep_get(ptep);
> +
> + if (likely(!pte_valid_cont(orig_pte)))
> + return __ptep_clear_flush_young(vma, addr, ptep);
> +
> + return contpte_ptep_clear_flush_young(vma, addr, ptep);
> +}
> +
> +#define __HAVE_ARCH_PTEP_SET_WRPROTECT
> +static inline void ptep_set_wrprotect(struct mm_struct *mm,
> + unsigned long addr, pte_t *ptep)
> +{
> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> + __ptep_set_wrprotect(mm, addr, ptep);
> +}
> +
> +#define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
> +static inline int ptep_set_access_flags(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep,
> + pte_t entry, int dirty)
> +{
> + pte_t orig_pte = __ptep_get(ptep);
> +
> + entry = pte_mknoncont(entry);
> +
> + if (likely(!pte_valid_cont(orig_pte)))
> + return __ptep_set_access_flags(vma, addr, ptep, entry, dirty);
> +
> + return contpte_ptep_set_access_flags(vma, addr, ptep, entry, dirty);
> +}
> +
> +#else /* CONFIG_ARM64_CONTPTE */
> +
> #define ptep_get __ptep_get
> #define set_pte __set_pte
> #define set_ptes __set_ptes
> @@ -1150,6 +1309,8 @@ void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte);
> #define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
> #define ptep_set_access_flags __ptep_set_access_flags
>
> +#endif /* CONFIG_ARM64_CONTPTE */
> +
> #endif /* !__ASSEMBLY__ */
>
> #endif /* __ASM_PGTABLE_H */
> diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
> index dbd1bc95967d..60454256945b 100644
> --- a/arch/arm64/mm/Makefile
> +++ b/arch/arm64/mm/Makefile
> @@ -3,6 +3,7 @@ obj-y := dma-mapping.o extable.o fault.o init.o \
> cache.o copypage.o flush.o \
> ioremap.o mmap.o pgd.o mmu.o \
> context.o proc.o pageattr.o fixmap.o
> +obj-$(CONFIG_ARM64_CONTPTE) += contpte.o
> obj-$(CONFIG_HUGETLB_PAGE) += hugetlbpage.o
> obj-$(CONFIG_PTDUMP_CORE) += ptdump.o
> obj-$(CONFIG_PTDUMP_DEBUGFS) += ptdump_debugfs.o
> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> new file mode 100644
> index 000000000000..bfb50e6b44c7
> --- /dev/null
> +++ b/arch/arm64/mm/contpte.c
> @@ -0,0 +1,283 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2023 ARM Ltd.
> + */
> +
> +#include <linux/mm.h>
> +#include <linux/export.h>
> +#include <asm/tlbflush.h>
> +
> +static inline bool mm_is_user(struct mm_struct *mm)
> +{
> + /*
> + * Don't attempt to apply the contig bit to kernel mappings, because
> + * dynamically adding/removing the contig bit can cause page faults.
> + * These racing faults are ok for user space, since they get serialized
> + * on the PTL. But kernel mappings can't tolerate faults.
> + */
> + return mm != &init_mm;
> +}

We also have the efi_mm as a non-user mm, though I don't think we manipulate
that while it is live, and I'm not sure if that needs any special handling.

> +static inline pte_t *contpte_align_down(pte_t *ptep)
> +{
> + return (pte_t *)(ALIGN_DOWN((unsigned long)ptep >> 3, CONT_PTES) << 3);

I think this can be:

static inline pte_t *contpte_align_down(pte_t *ptep)
{
return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
}

> +
> +static void contpte_convert(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, pte_t pte)
> +{
> + 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));
> +
> + 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);
> + }

Not a big deal either way, but I wonder if it makes more sense to accumulate
the 'ptent' dirty/young values, then modify 'pte' once, i.e.

bool dirty = false, young = false;

for (...) {
pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
dirty |= pte_dirty(ptent);
young |= pte_young(ptent);
}

if (dirty)
pte_mkdirty(pte);
if (young)
pte_mkyoung(pte);

I suspect that might generate slightly better code, but I'm also happy with the
current form if people thnk that's more legible (I have no strong feelings
either way).

> +
> + __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
> +
> + __set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
> +}
> +
> +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 just check that the mm is user space.
> + */
> +
> + if (!mm_is_user(mm))
> + return;

Nit: normally we don't put a line gap between a comment block and the
associated block of code.

> +
> + pte = pte_mknoncont(pte);
> + contpte_convert(mm, addr, ptep, pte);
> +}
> +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.
> + */

Nit: s/guarranteed/guaranteed/

> +
> + 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.
> + */
> +
> + pgprot_t orig_prot;
> + unsigned long pfn;
> + pte_t orig_pte;
> + pgprot_t prot;
> + pte_t *ptep;
> + pte_t pte;
> + 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);

I'm struggling to convince myself that this is safe in general, as it really
depends on how the caller will use this value. Which caller(s) actually care
about the access/dirty bits, given those could change at any time anyway?

I took a quick scan, and AFAICT:

* For perf_get_pgtable_size(), we only care about whether the entry is valid
and has the contig bit set. We could clean that up with a new interface, e.g.
something like a new ptep_get_size_lockless().

* For gup_pte_range(), I'm not sure we actually need the access/dirty bits when
we look at the pte to start with, since we only care where we can logically
write to the page at that point.

I see that we later follow up with:

with pte_val(pte) != pte_val(ptep_get(ptep)))

... is that why we need ptep_get_lockless() to accumulate the access/dirty
bits? So that shape of lockless-try...locked-compare sequence works?

* For huge_pte_alloc(), arm64 doesn't select CONFIG_ARCH_WANT_GENERAL_HUGETLB,
so this doesn' seem to matter.

* For __collapse_huge_page_swapin(), we only care if the pte is a swap pte,
which means the pte isn't valid, and we'll return the orig_pte as-is anyway.

* For pte_range_none() the access/dirty bits don't matter.

* For handle_pte_fault() I think we have the same shape of
lockless-try...locked-compare sequence as for gup_pte_range(), where we don't
care about the acess/dirty bits before we reach the locked compare step.

* For ptdump_pte_entry() I think it's arguable that we should continue to
report the access/dirty bits separately for each PTE, as we have done until
now, to give an accurate representation of the contents of the translation
tables.

* For swap_vma_readahead() and unuse_pte_range() we only care if the PTE is a
swap entry, the access/dirty bits don't matter.

So AFAICT this only really matters for gup_pte_range() and handle_pte_fault(),
and IIUC that's only so that the locklessly-loaded pte value can be compared
with a subsequently locked-loaded entry (for which the access/dirty bits will
be accumulated). Have I understood that correctly?

If so, I wonder if we could instead do that comparison modulo the access/dirty
bits, and leave ptep_get_lockless() only reading a single entry?

Thanks,
Mark.

> +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;
> + unsigned long pfn;
> + pgprot_t prot;
> +
> + /*
> + * The set_ptes() spec guarantees that when nr > 1, the initial state of
> + * all ptes is not-present. Therefore we never need to unfold or
> + * otherwise invalidate a range before we set the new ptes.
> + * contpte_set_ptes() should never be called for nr < 2.
> + */
> + VM_WARN_ON(nr == 1);
> +
> + if (!mm_is_user(mm))
> + return __set_ptes(mm, addr, ptep, pte, nr);
> +
> + end = addr + (nr << PAGE_SHIFT);
> + pfn = pte_pfn(pte);
> + prot = pte_pgprot(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);
> +
> + __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 young = 0;
> + int i;
> +
> + 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)
> +{
> + unsigned long start_addr;
> + pte_t orig_pte;
> + int i;
> +
> + /*
> + * Gather the access/dirty bits for the contiguous range. If nothing has
> + * changed, its a noop.
> + */
> + orig_pte = pte_mknoncont(ptep_get(ptep));
> + if (pte_val(orig_pte) == pte_val(entry))
> + return 0;
> +
> + /*
> + * We can fix up access/dirty bits without having to unfold the contig
> + * range. But if the write bit is changing, we must unfold.
> + */
> + 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);
> + }
> +
> + return 1;
> +}
> +EXPORT_SYMBOL(contpte_ptep_set_access_flags);
> --
> 2.25.1
>