mm/mapping_dirty_helpers.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/pagewalk.h>
 #include <linux/hugetlb.h>
 #include <linux/bitops.h>
 #include <linux/mmu_notifier.h>
 #include <linux/mm_inline.h>
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>

 /**
  * struct wp_walk - Private struct for pagetable walk callbacks
  * @range: Range for mmu notifiers
  * @tlbflush_start: Address of first modified pte
  * @tlbflush_end: Address of last modified pte + 1
  * @total: Total number of modified ptes
  */
 struct wp_walk {
 	struct mmu_notifier_range range;
 	unsigned long tlbflush_start;
 	unsigned long tlbflush_end;
 	unsigned long total;
 };

 /**
  * wp_pte - Write-protect a pte
  * @pte: Pointer to the pte
  * @addr: The start of protecting virtual address
  * @end: The end of protecting virtual address
  * @walk: pagetable walk callback argument
  *
  * The function write-protects a pte and records the range in
  * virtual address space of touched ptes for efficient range TLB flushes.
  */
 static int wp_pte(pte_t *pte, unsigned long addr, unsigned long end,
 		  struct mm_walk *walk)
 {
 	struct wp_walk *wpwalk = walk->private;
 	pte_t ptent = *pte;

 	if (pte_write(ptent)) {
 		pte_t old_pte = ptep_modify_prot_start(walk->vma, addr, pte);

 		ptent = pte_wrprotect(old_pte);
 		ptep_modify_prot_commit(walk->vma, addr, pte, old_pte, ptent);
 		wpwalk->total++;
 		wpwalk->tlbflush_start = min(wpwalk->tlbflush_start, addr);
 		wpwalk->tlbflush_end = max(wpwalk->tlbflush_end,
 					   addr + PAGE_SIZE);
 	}

 	return 0;
 }

 /**
  * struct clean_walk - Private struct for the clean_record_pte function.
  * @base: struct wp_walk we derive from
  * @bitmap_pgoff: Address_space Page offset of the first bit in @bitmap
  * @bitmap: Bitmap with one bit for each page offset in the address_space range
  * covered.
  * @start: Address_space page offset of first modified pte relative
  * to @bitmap_pgoff
  * @end: Address_space page offset of last modified pte relative
  * to @bitmap_pgoff
  */
 struct clean_walk {
 	struct wp_walk base;
 	pgoff_t bitmap_pgoff;
 	unsigned long *bitmap;
 	pgoff_t start;
 	pgoff_t end;
 };

 #define to_clean_walk(_wpwalk) container_of(_wpwalk, struct clean_walk, base)

 /**
  * clean_record_pte - Clean a pte and record its address space offset in a
  * bitmap
  * @pte: Pointer to the pte
  * @addr: The start of virtual address to be clean
  * @end: The end of virtual address to be clean
  * @walk: pagetable walk callback argument
  *
  * The function cleans a pte and records the range in
  * virtual address space of touched ptes for efficient TLB flushes.
  * It also records dirty ptes in a bitmap representing page offsets
  * in the address_space, as well as the first and last of the bits
  * touched.
  */
 static int clean_record_pte(pte_t *pte, unsigned long addr,
 			    unsigned long end, struct mm_walk *walk)
 {
 	struct wp_walk *wpwalk = walk->private;
 	struct clean_walk *cwalk = to_clean_walk(wpwalk);
 	pte_t ptent = *pte;

 	if (pte_dirty(ptent)) {
 		pgoff_t pgoff = ((addr - walk->vma->vm_start) >> PAGE_SHIFT) +
 			walk->vma->vm_pgoff - cwalk->bitmap_pgoff;
 		pte_t old_pte = ptep_modify_prot_start(walk->vma, addr, pte);

 		ptent = pte_mkclean(old_pte);
 		ptep_modify_prot_commit(walk->vma, addr, pte, old_pte, ptent);

 		wpwalk->total++;
 		wpwalk->tlbflush_start = min(wpwalk->tlbflush_start, addr);
 		wpwalk->tlbflush_end = max(wpwalk->tlbflush_end,
 					   addr + PAGE_SIZE);

 		__set_bit(pgoff, cwalk->bitmap);
 		cwalk->start = min(cwalk->start, pgoff);
 		cwalk->end = max(cwalk->end, pgoff + 1);
 	}

 	return 0;
 }

 /*
  * wp_clean_pmd_entry - The pagewalk pmd callback.
  *
  * Dirty-tracking should take place on the PTE level, so
  * WARN() if encountering a dirty huge pmd.
  * Furthermore, never split huge pmds, since that currently
  * causes dirty info loss. The pagefault handler should do
  * that if needed.
  */
 static int wp_clean_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long end,
 			      struct mm_walk *walk)
 {
 	pmd_t pmdval = pmdp_get_lockless(pmd);

 	if (!pmd_trans_unstable(&pmdval))
 		return 0;

 	if (pmd_none(pmdval)) {
 		walk->action = ACTION_AGAIN;
 		return 0;
 	}

 	/* Huge pmd, present or migrated */
 	walk->action = ACTION_CONTINUE;
 	if (pmd_trans_huge(pmdval) || pmd_devmap(pmdval))
 		WARN_ON(pmd_write(pmdval) || pmd_dirty(pmdval));

 	return 0;
 }

 /*
  * wp_clean_pud_entry - The pagewalk pud callback.
  *
  * Dirty-tracking should take place on the PTE level, so
  * WARN() if encountering a dirty huge puds.
  * Furthermore, never split huge puds, since that currently
  * causes dirty info loss. The pagefault handler should do
  * that if needed.
  */
 static int wp_clean_pud_entry(pud_t *pud, unsigned long addr, unsigned long end,
 			      struct mm_walk *walk)
 {
 	pud_t pudval = READ_ONCE(*pud);

 	if (!pud_trans_unstable(&pudval))
 		return 0;

 	if (pud_none(pudval)) {
 		walk->action = ACTION_AGAIN;
 		return 0;
 	}

 #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 	/* Huge pud */
 	walk->action = ACTION_CONTINUE;
 	if (pud_trans_huge(pudval) || pud_devmap(pudval))
 		WARN_ON(pud_write(pudval) || pud_dirty(pudval));
 #endif

 	return 0;
 }

 /*
  * wp_clean_pre_vma - The pagewalk pre_vma callback.
  *
  * The pre_vma callback performs the cache flush, stages the tlb flush
  * and calls the necessary mmu notifiers.
  */
 static int wp_clean_pre_vma(unsigned long start, unsigned long end,
 			    struct mm_walk *walk)
 {
 	struct wp_walk *wpwalk = walk->private;

 	wpwalk->tlbflush_start = end;
 	wpwalk->tlbflush_end = start;

 	mmu_notifier_range_init(&wpwalk->range, MMU_NOTIFY_PROTECTION_PAGE, 0,
 				walk->vma, walk->mm, start, end);
 	mmu_notifier_invalidate_range_start(&wpwalk->range);
 	flush_cache_range(walk->vma, start, end);

 	/*
 	 * We're not using tlb_gather_mmu() since typically
 	 * only a small subrange of PTEs are affected, whereas
 	 * tlb_gather_mmu() records the full range.
 	 */
 	inc_tlb_flush_pending(walk->mm);

 	return 0;
 }

 /*
  * wp_clean_post_vma - The pagewalk post_vma callback.
  *
  * The post_vma callback performs the tlb flush and calls necessary mmu
  * notifiers.
  */
 static void wp_clean_post_vma(struct mm_walk *walk)
 {
 	struct wp_walk *wpwalk = walk->private;

 	if (mm_tlb_flush_nested(walk->mm))
 		flush_tlb_range(walk->vma, wpwalk->range.start,
 				wpwalk->range.end);
 	else if (wpwalk->tlbflush_end > wpwalk->tlbflush_start)
 		flush_tlb_range(walk->vma, wpwalk->tlbflush_start,
 				wpwalk->tlbflush_end);

 	mmu_notifier_invalidate_range_end(&wpwalk->range);
 	dec_tlb_flush_pending(walk->mm);
 }

 /*
  * wp_clean_test_walk - The pagewalk test_walk callback.
  *
  * Won't perform dirty-tracking on COW, read-only or HUGETLB vmas.
  */
 static int wp_clean_test_walk(unsigned long start, unsigned long end,
 			      struct mm_walk *walk)
 {
 	unsigned long vm_flags = READ_ONCE(walk->vma->vm_flags);

 	/* Skip non-applicable VMAs */
 	if ((vm_flags & (VM_SHARED | VM_MAYWRITE | VM_HUGETLB)) !=
 	    (VM_SHARED | VM_MAYWRITE))
 		return 1;

 	return 0;
 }

 static const struct mm_walk_ops clean_walk_ops = {
 	.pte_entry = clean_record_pte,
 	.pmd_entry = wp_clean_pmd_entry,
 	.pud_entry = wp_clean_pud_entry,
 	.test_walk = wp_clean_test_walk,
 	.pre_vma = wp_clean_pre_vma,
 	.post_vma = wp_clean_post_vma
 };

 static const struct mm_walk_ops wp_walk_ops = {
 	.pte_entry = wp_pte,
 	.pmd_entry = wp_clean_pmd_entry,
 	.pud_entry = wp_clean_pud_entry,
 	.test_walk = wp_clean_test_walk,
 	.pre_vma = wp_clean_pre_vma,
 	.post_vma = wp_clean_post_vma
 };

 /**
  * wp_shared_mapping_range - Write-protect all ptes in an address space range
  * @mapping: The address_space we want to write protect
  * @first_index: The first page offset in the range
  * @nr: Number of incremental page offsets to cover
  *
  * Note: This function currently skips transhuge page-table entries, since
  * it's intended for dirty-tracking on the PTE level. It will warn on
  * encountering transhuge write-enabled entries, though, and can easily be
  * extended to handle them as well.
  *
  * Return: The number of ptes actually write-protected. Note that
  * already write-protected ptes are not counted.
  */
 unsigned long wp_shared_mapping_range(struct address_space *mapping,
 				      pgoff_t first_index, pgoff_t nr)
 {
 	struct wp_walk wpwalk = { .total = 0 };

 	i_mmap_lock_read(mapping);
 	WARN_ON(walk_page_mapping(mapping, first_index, nr, &wp_walk_ops,
 				  &wpwalk));
 	i_mmap_unlock_read(mapping);

 	return wpwalk.total;
 }
 EXPORT_SYMBOL_GPL(wp_shared_mapping_range);

 /**
  * clean_record_shared_mapping_range - Clean and record all ptes in an
  * address space range
  * @mapping: The address_space we want to clean
  * @first_index: The first page offset in the range
  * @nr: Number of incremental page offsets to cover
  * @bitmap_pgoff: The page offset of the first bit in @bitmap
  * @bitmap: Pointer to a bitmap of at least @nr bits. The bitmap needs to
  * cover the whole range @first_index..@first_index + @nr.
  * @start: Pointer to number of the first set bit in @bitmap.
  * is modified as new bits are set by the function.
  * @end: Pointer to the number of the last set bit in @bitmap.
  * none set. The value is modified as new bits are set by the function.
  *
  * Note: When this function returns there is no guarantee that a CPU has
  * not already dirtied new ptes. However it will not clean any ptes not
  * reported in the bitmap. The guarantees are as follows:
  * a) All ptes dirty when the function starts executing will end up recorded
  *    in the bitmap.
  * b) All ptes dirtied after that will either remain dirty, be recorded in the
  *    bitmap or both.
  *
  * If a caller needs to make sure all dirty ptes are picked up and none
  * additional are added, it first needs to write-protect the address-space
  * range and make sure new writers are blocked in page_mkwrite() or
  * pfn_mkwrite(). And then after a TLB flush following the write-protection
  * pick up all dirty bits.
  *
  * This function currently skips transhuge page-table entries, since
  * it's intended for dirty-tracking on the PTE level. It will warn on
  * encountering transhuge dirty entries, though, and can easily be extended
  * to handle them as well.
  *
  * Return: The number of dirty ptes actually cleaned.
  */
 unsigned long clean_record_shared_mapping_range(struct address_space *mapping,
 						pgoff_t first_index, pgoff_t nr,
 						pgoff_t bitmap_pgoff,
 						unsigned long *bitmap,
 						pgoff_t *start,
 						pgoff_t *end)
 {
 	bool none_set = (*start >= *end);
 	struct clean_walk cwalk = {
 		.base = { .total = 0 },
 		.bitmap_pgoff = bitmap_pgoff,
 		.bitmap = bitmap,
 		.start = none_set ? nr : *start,
 		.end = none_set ? 0 : *end,
 	};

 	i_mmap_lock_read(mapping);
 	WARN_ON(walk_page_mapping(mapping, first_index, nr, &clean_walk_ops,
 				  &cwalk.base));
 	i_mmap_unlock_read(mapping);

 	*start = cwalk.start;
 	*end = cwalk.end;

 	return cwalk.base.total;
 }
 EXPORT_SYMBOL_GPL(clean_record_shared_mapping_range);
	// SPDX-License-Identifier: GPL-2.0
	#include <linux/pagewalk.h>
	#include <linux/hugetlb.h>
	#include <linux/bitops.h>
	#include <linux/mmu_notifier.h>
	#include <linux/mm_inline.h>
	#include <asm/cacheflush.h>
	#include <asm/tlbflush.h>

	/**
	* struct wp_walk - Private struct for pagetable walk callbacks
	* @range: Range for mmu notifiers
	* @tlbflush_start: Address of first modified pte
	* @tlbflush_end: Address of last modified pte + 1
	* @total: Total number of modified ptes
	*/
	struct wp_walk {
	struct mmu_notifier_range range;
	unsigned long tlbflush_start;
	unsigned long tlbflush_end;
	unsigned long total;
	};

	/**
	* wp_pte - Write-protect a pte
	* @pte: Pointer to the pte
	* @addr: The start of protecting virtual address
	* @end: The end of protecting virtual address
	* @walk: pagetable walk callback argument
	*
	* The function write-protects a pte and records the range in
	* virtual address space of touched ptes for efficient range TLB flushes.
	*/
	static int wp_pte(pte_t *pte, unsigned long addr, unsigned long end,
	struct mm_walk *walk)
	{
	struct wp_walk *wpwalk = walk->private;
	pte_t ptent = *pte;

	if (pte_write(ptent)) {
	pte_t old_pte = ptep_modify_prot_start(walk->vma, addr, pte);

	ptent = pte_wrprotect(old_pte);
	ptep_modify_prot_commit(walk->vma, addr, pte, old_pte, ptent);
	wpwalk->total++;
	wpwalk->tlbflush_start = min(wpwalk->tlbflush_start, addr);
	wpwalk->tlbflush_end = max(wpwalk->tlbflush_end,
	addr + PAGE_SIZE);
	}

	return 0;
	}

	/**
	* struct clean_walk - Private struct for the clean_record_pte function.
	* @base: struct wp_walk we derive from
	* @bitmap_pgoff: Address_space Page offset of the first bit in @bitmap
	* @bitmap: Bitmap with one bit for each page offset in the address_space range
	* covered.
	* @start: Address_space page offset of first modified pte relative
	* to @bitmap_pgoff
	* @end: Address_space page offset of last modified pte relative
	* to @bitmap_pgoff
	*/
	struct clean_walk {
	struct wp_walk base;
	pgoff_t bitmap_pgoff;
	unsigned long *bitmap;
	pgoff_t start;
	pgoff_t end;
	};

	#define to_clean_walk(_wpwalk) container_of(_wpwalk, struct clean_walk, base)

	/**
	* clean_record_pte - Clean a pte and record its address space offset in a
	* bitmap
	* @pte: Pointer to the pte
	* @addr: The start of virtual address to be clean
	* @end: The end of virtual address to be clean
	* @walk: pagetable walk callback argument
	*
	* The function cleans a pte and records the range in
	* virtual address space of touched ptes for efficient TLB flushes.
	* It also records dirty ptes in a bitmap representing page offsets
	* in the address_space, as well as the first and last of the bits
	* touched.
	*/
	static int clean_record_pte(pte_t *pte, unsigned long addr,
	unsigned long end, struct mm_walk *walk)
	{
	struct wp_walk *wpwalk = walk->private;
	struct clean_walk *cwalk = to_clean_walk(wpwalk);
	pte_t ptent = *pte;

	if (pte_dirty(ptent)) {
	pgoff_t pgoff = ((addr - walk->vma->vm_start) >> PAGE_SHIFT) +
	walk->vma->vm_pgoff - cwalk->bitmap_pgoff;
	pte_t old_pte = ptep_modify_prot_start(walk->vma, addr, pte);

	ptent = pte_mkclean(old_pte);
	ptep_modify_prot_commit(walk->vma, addr, pte, old_pte, ptent);

	wpwalk->total++;
	wpwalk->tlbflush_start = min(wpwalk->tlbflush_start, addr);
	wpwalk->tlbflush_end = max(wpwalk->tlbflush_end,
	addr + PAGE_SIZE);

	__set_bit(pgoff, cwalk->bitmap);
	cwalk->start = min(cwalk->start, pgoff);
	cwalk->end = max(cwalk->end, pgoff + 1);
	}

	return 0;
	}

	/*
	* wp_clean_pmd_entry - The pagewalk pmd callback.
	*
	* Dirty-tracking should take place on the PTE level, so
	* WARN() if encountering a dirty huge pmd.
	* Furthermore, never split huge pmds, since that currently
	* causes dirty info loss. The pagefault handler should do
	* that if needed.
	*/
	static int wp_clean_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long end,
	struct mm_walk *walk)
	{
	pmd_t pmdval = pmdp_get_lockless(pmd);

	if (!pmd_trans_unstable(&pmdval))
	return 0;

	if (pmd_none(pmdval)) {
	walk->action = ACTION_AGAIN;
	return 0;
	}

	/* Huge pmd, present or migrated */
	walk->action = ACTION_CONTINUE;
	if (pmd_trans_huge(pmdval) \|\| pmd_devmap(pmdval))
	WARN_ON(pmd_write(pmdval) \|\| pmd_dirty(pmdval));

	return 0;
	}

	/*
	* wp_clean_pud_entry - The pagewalk pud callback.
	*
	* Dirty-tracking should take place on the PTE level, so
	* WARN() if encountering a dirty huge puds.
	* Furthermore, never split huge puds, since that currently
	* causes dirty info loss. The pagefault handler should do
	* that if needed.
	*/
	static int wp_clean_pud_entry(pud_t *pud, unsigned long addr, unsigned long end,
	struct mm_walk *walk)
	{
	pud_t pudval = READ_ONCE(*pud);

	if (!pud_trans_unstable(&pudval))
	return 0;

	if (pud_none(pudval)) {
	walk->action = ACTION_AGAIN;
	return 0;
	}

	#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
	/* Huge pud */
	walk->action = ACTION_CONTINUE;
	if (pud_trans_huge(pudval) \|\| pud_devmap(pudval))
	WARN_ON(pud_write(pudval) \|\| pud_dirty(pudval));
	#endif

	return 0;
	}

	/*
	* wp_clean_pre_vma - The pagewalk pre_vma callback.
	*
	* The pre_vma callback performs the cache flush, stages the tlb flush
	* and calls the necessary mmu notifiers.
	*/
	static int wp_clean_pre_vma(unsigned long start, unsigned long end,
	struct mm_walk *walk)
	{
	struct wp_walk *wpwalk = walk->private;

	wpwalk->tlbflush_start = end;
	wpwalk->tlbflush_end = start;

	mmu_notifier_range_init(&wpwalk->range, MMU_NOTIFY_PROTECTION_PAGE, 0,
	walk->vma, walk->mm, start, end);
	mmu_notifier_invalidate_range_start(&wpwalk->range);
	flush_cache_range(walk->vma, start, end);

	/*
	* We're not using tlb_gather_mmu() since typically
	* only a small subrange of PTEs are affected, whereas
	* tlb_gather_mmu() records the full range.
	*/
	inc_tlb_flush_pending(walk->mm);

	return 0;
	}

	/*
	* wp_clean_post_vma - The pagewalk post_vma callback.
	*
	* The post_vma callback performs the tlb flush and calls necessary mmu
	* notifiers.
	*/
	static void wp_clean_post_vma(struct mm_walk *walk)
	{
	struct wp_walk *wpwalk = walk->private;

	if (mm_tlb_flush_nested(walk->mm))
	flush_tlb_range(walk->vma, wpwalk->range.start,
	wpwalk->range.end);
	else if (wpwalk->tlbflush_end > wpwalk->tlbflush_start)
	flush_tlb_range(walk->vma, wpwalk->tlbflush_start,
	wpwalk->tlbflush_end);

	mmu_notifier_invalidate_range_end(&wpwalk->range);
	dec_tlb_flush_pending(walk->mm);
	}

	/*
	* wp_clean_test_walk - The pagewalk test_walk callback.
	*
	* Won't perform dirty-tracking on COW, read-only or HUGETLB vmas.
	*/
	static int wp_clean_test_walk(unsigned long start, unsigned long end,
	struct mm_walk *walk)
	{
	unsigned long vm_flags = READ_ONCE(walk->vma->vm_flags);

	/* Skip non-applicable VMAs */
	if ((vm_flags & (VM_SHARED \| VM_MAYWRITE \| VM_HUGETLB)) !=
	(VM_SHARED \| VM_MAYWRITE))
	return 1;

	return 0;
	}

	static const struct mm_walk_ops clean_walk_ops = {
	.pte_entry = clean_record_pte,
	.pmd_entry = wp_clean_pmd_entry,
	.pud_entry = wp_clean_pud_entry,
	.test_walk = wp_clean_test_walk,
	.pre_vma = wp_clean_pre_vma,
	.post_vma = wp_clean_post_vma
	};

	static const struct mm_walk_ops wp_walk_ops = {
	.pte_entry = wp_pte,
	.pmd_entry = wp_clean_pmd_entry,
	.pud_entry = wp_clean_pud_entry,
	.test_walk = wp_clean_test_walk,
	.pre_vma = wp_clean_pre_vma,
	.post_vma = wp_clean_post_vma
	};

	/**
	* wp_shared_mapping_range - Write-protect all ptes in an address space range
	* @mapping: The address_space we want to write protect
	* @first_index: The first page offset in the range
	* @nr: Number of incremental page offsets to cover
	*
	* Note: This function currently skips transhuge page-table entries, since
	* it's intended for dirty-tracking on the PTE level. It will warn on
	* encountering transhuge write-enabled entries, though, and can easily be
	* extended to handle them as well.
	*
	* Return: The number of ptes actually write-protected. Note that
	* already write-protected ptes are not counted.
	*/
	unsigned long wp_shared_mapping_range(struct address_space *mapping,
	pgoff_t first_index, pgoff_t nr)
	{
	struct wp_walk wpwalk = { .total = 0 };

	i_mmap_lock_read(mapping);
	WARN_ON(walk_page_mapping(mapping, first_index, nr, &wp_walk_ops,
	&wpwalk));
	i_mmap_unlock_read(mapping);

	return wpwalk.total;
	}
	EXPORT_SYMBOL_GPL(wp_shared_mapping_range);

	/**
	* clean_record_shared_mapping_range - Clean and record all ptes in an
	* address space range
	* @mapping: The address_space we want to clean
	* @first_index: The first page offset in the range
	* @nr: Number of incremental page offsets to cover
	* @bitmap_pgoff: The page offset of the first bit in @bitmap
	* @bitmap: Pointer to a bitmap of at least @nr bits. The bitmap needs to
	* cover the whole range @first_index..@first_index + @nr.
	* @start: Pointer to number of the first set bit in @bitmap.
	* is modified as new bits are set by the function.
	* @end: Pointer to the number of the last set bit in @bitmap.
	* none set. The value is modified as new bits are set by the function.
	*
	* Note: When this function returns there is no guarantee that a CPU has
	* not already dirtied new ptes. However it will not clean any ptes not
	* reported in the bitmap. The guarantees are as follows:
	* a) All ptes dirty when the function starts executing will end up recorded
	* in the bitmap.
	* b) All ptes dirtied after that will either remain dirty, be recorded in the
	* bitmap or both.
	*
	* If a caller needs to make sure all dirty ptes are picked up and none
	* additional are added, it first needs to write-protect the address-space
	* range and make sure new writers are blocked in page_mkwrite() or
	* pfn_mkwrite(). And then after a TLB flush following the write-protection
	* pick up all dirty bits.
	*
	* This function currently skips transhuge page-table entries, since
	* it's intended for dirty-tracking on the PTE level. It will warn on
	* encountering transhuge dirty entries, though, and can easily be extended
	* to handle them as well.
	*
	* Return: The number of dirty ptes actually cleaned.
	*/
	unsigned long clean_record_shared_mapping_range(struct address_space *mapping,
	pgoff_t first_index, pgoff_t nr,
	pgoff_t bitmap_pgoff,
	unsigned long *bitmap,
	pgoff_t *start,
	pgoff_t *end)
	{
	bool none_set = (start >= end);
	struct clean_walk cwalk = {
	.base = { .total = 0 },
	.bitmap_pgoff = bitmap_pgoff,
	.bitmap = bitmap,
	.start = none_set ? nr : *start,
	.end = none_set ? 0 : *end,
	};

	i_mmap_lock_read(mapping);
	WARN_ON(walk_page_mapping(mapping, first_index, nr, &clean_walk_ops,
	&cwalk.base));
	i_mmap_unlock_read(mapping);

	*start = cwalk.start;
	*end = cwalk.end;

	return cwalk.base.total;
	}
	EXPORT_SYMBOL_GPL(clean_record_shared_mapping_range);