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mm, thp: Do not make page table dirty unconditionally in touch_p[mu]d()

Currently, we unconditionally make page table dirty in touch_pmd().
It may result in false-positive can_follow_write_pmd().

We may avoid the situation, if we would only make the page table entry
dirty if caller asks for write access -- FOLL_WRITE.

The patch also changes touch_pud() in the same way.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Kirill A. Shutemov authored and torvalds committed Nov 27, 2017
1 parent 4fbd8d1 commit a8f97366452ed491d13cf1e44241bc0b5740b1f0
Showing with 13 additions and 23 deletions.
  1. +13 −23 mm/huge_memory.c
@@ -842,20 +842,15 @@ EXPORT_SYMBOL_GPL(vmf_insert_pfn_pud);
#endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */

static void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
pmd_t *pmd)
pmd_t *pmd, int flags)
{
pmd_t _pmd;

/*
* We should set the dirty bit only for FOLL_WRITE but for now
* the dirty bit in the pmd is meaningless. And if the dirty
* bit will become meaningful and we'll only set it with
* FOLL_WRITE, an atomic set_bit will be required on the pmd to
* set the young bit, instead of the current set_pmd_at.
*/
_pmd = pmd_mkyoung(pmd_mkdirty(*pmd));
_pmd = pmd_mkyoung(*pmd);
if (flags & FOLL_WRITE)
_pmd = pmd_mkdirty(_pmd);
if (pmdp_set_access_flags(vma, addr & HPAGE_PMD_MASK,
pmd, _pmd, 1))
pmd, _pmd, flags & FOLL_WRITE))
update_mmu_cache_pmd(vma, addr, pmd);
}

@@ -884,7 +879,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
return NULL;

if (flags & FOLL_TOUCH)
touch_pmd(vma, addr, pmd);
touch_pmd(vma, addr, pmd, flags);

/*
* device mapped pages can only be returned if the
@@ -995,20 +990,15 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,

#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
static void touch_pud(struct vm_area_struct *vma, unsigned long addr,
pud_t *pud)
pud_t *pud, int flags)
{
pud_t _pud;

/*
* We should set the dirty bit only for FOLL_WRITE but for now
* the dirty bit in the pud is meaningless. And if the dirty
* bit will become meaningful and we'll only set it with
* FOLL_WRITE, an atomic set_bit will be required on the pud to
* set the young bit, instead of the current set_pud_at.
*/
_pud = pud_mkyoung(pud_mkdirty(*pud));
_pud = pud_mkyoung(*pud);
if (flags & FOLL_WRITE)
_pud = pud_mkdirty(_pud);
if (pudp_set_access_flags(vma, addr & HPAGE_PUD_MASK,
pud, _pud, 1))
pud, _pud, flags & FOLL_WRITE))
update_mmu_cache_pud(vma, addr, pud);
}

@@ -1031,7 +1021,7 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
return NULL;

if (flags & FOLL_TOUCH)
touch_pud(vma, addr, pud);
touch_pud(vma, addr, pud, flags);

/*
* device mapped pages can only be returned if the
@@ -1424,7 +1414,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
page = pmd_page(*pmd);
VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
if (flags & FOLL_TOUCH)
touch_pmd(vma, addr, pmd);
touch_pmd(vma, addr, pmd, flags);
if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
/*
* We don't mlock() pte-mapped THPs. This way we can avoid

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