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 <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Hugh Dickins <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
torvalds · Nov 27, 2017 · a8f9736 · a8f9736
1 parent 4fbd8d1
commit a8f9736
Showing 1 changed file with 13 additions and 23 deletions.
diff --git a/mm/huge_memory.c b/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