Re: [PATCH v4 14/16] arm64/mm: Wire up PTE_CONT for user mappings

[Alexandre Ghiti]

Hi Ryan,

On 18/12/2023 11:50, 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. 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 guarantee 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.
>
> Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>   arch/arm64/Kconfig               |  10 +-
>   arch/arm64/include/asm/pgtable.h | 184 +++++++++++++++
>   arch/arm64/mm/Makefile           |   1 +
>   arch/arm64/mm/contpte.c          | 388 +++++++++++++++++++++++++++++++
>   4 files changed, 582 insertions(+), 1 deletion(-)
>   create mode 100644 arch/arm64/mm/contpte.c
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7b071a00425d..de76e484ff3a 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2209,6 +2209,15 @@ 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"
> @@ -2318,4 +2327,3 @@ endmenu # "CPU Power Management"
>   source "drivers/acpi/Kconfig"
>   
>   source "arch/arm64/kvm/Kconfig"
> -
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 6930c14f062f..e64120452301 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
> @@ -1116,6 +1120,184 @@ extern void ptep_modify_prot_commit(struct vm_area_struct *vma,
>   				    unsigned long addr, pte_t *ptep,
>   				    pte_t old_pte, pte_t new_pte);
>   
> +#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_fold(struct mm_struct *mm, unsigned long addr,
> +				pte_t *ptep, pte_t pte);
> +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_fold(struct mm_struct *mm, unsigned long addr,
> +					pte_t *ptep, pte_t pte)
> +{
> +	/*
> +	 * Only bother trying if both the virtual and physical addresses are
> +	 * aligned and correspond to the last entry in a contig range. The core
> +	 * code mostly modifies ranges from low to high, so this is the likely
> +	 * the last modification in the contig range, so a good time to fold.
> +	 * We can't fold special mappings, because there is no associated folio.
> +	 */
> +
> +	const unsigned long contmask = CONT_PTES - 1;
> +	bool valign = (((unsigned long)ptep >> 3) & contmask) == contmask;
> +	bool palign = (pte_pfn(pte) & contmask) == contmask;
> +
> +	if (valign && palign &&
> +	    pte_valid(pte) && !pte_cont(pte) && !pte_special(pte))
> +		__contpte_try_fold(mm, addr, ptep, pte);
> +}
> +
> +static inline void contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> +					pte_t *ptep, pte_t pte)
> +{
> +	if (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 (!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 (!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 or fold 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);
> +
> +	if (nr == 1) {
> +		contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> +		__set_ptes(mm, addr, ptep, pte, 1);
> +		contpte_try_fold(mm, addr, ptep, pte);
> +	} 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 (!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 (!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);
> +	contpte_try_fold(mm, addr, ptep, __ptep_get(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 (!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
> @@ -1131,6 +1313,8 @@ extern void ptep_modify_prot_commit(struct vm_area_struct *vma,
>   #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..69c36749dd98
> --- /dev/null
> +++ b/arch/arm64/mm/contpte.c
> @@ -0,0 +1,388 @@
> +// 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;
> +}
> +
> +static inline pte_t *contpte_align_down(pte_t *ptep)
> +{
> +	return (pte_t *)(ALIGN_DOWN((unsigned long)ptep >> 3, CONT_PTES) << 3);
> +}
> +
> +static void ptep_clear_flush_range(struct mm_struct *mm, unsigned long addr,
> +				pte_t *ptep, int nr)
> +{
> +	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
> +	unsigned long start_addr = addr;
> +	int i;
> +
> +	for (i = 0; i < nr; i++, ptep++, addr += PAGE_SIZE)
> +		__pte_clear(mm, addr, ptep);
> +
> +	__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
> +}
> +
> +static bool ptep_any_valid(pte_t *ptep, int nr)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr; i++, ptep++) {
> +		if (pte_valid(__ptep_get(ptep)))
> +			return true;
> +	}
> +
> +	return false;
> +}
> +
> +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);
> +	}
> +
> +	__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.
> +	 */
> +
> +	unsigned long folio_saddr;
> +	unsigned long folio_eaddr;
> +	unsigned long cont_saddr;
> +	unsigned long cont_eaddr;
> +	struct folio *folio;
> +	struct page *page;
> +	unsigned long pfn;
> +	pte_t *orig_ptep;
> +	pgprot_t prot;
> +	pte_t subpte;
> +	int i;
> +
> +	if (!mm_is_user(mm))
> +		return;
> +
> +	page = pte_page(pte);
> +	folio = page_folio(page);
> +	folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
> +	folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
> +	cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> +	cont_eaddr = cont_saddr + CONT_PTE_SIZE;
> +
> +	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;
> +	}
> +
> +	pte = pte_mkcont(pte);
> +	contpte_convert(mm, addr, orig_ptep, pte);
> +}
> +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 just check that the mm is user space.
> +	 */
> +
> +	if (!mm_is_user(mm))
> +		return;
> +
> +	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.
> +	 */
> +
> +	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);
> +
> +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;
> +	pte_t orig_pte;
> +
> +	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);
> +
> +		/*
> +		 * 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 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/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);


entry was pte_mknoncont() in ptep_set_access_flags() so here you lose 
the contpte range, is that intentional? Or am I mistaken?


> +
> +		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);