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Subject[PATCH 4.9 03/38] mm, thp: Do not make page table dirty unconditionally in touch_p[mu]d()
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

commit a8f97366452ed491d13cf1e44241bc0b5740b1f0 upstream.

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>
[Salvatore Bonaccorso: backport for 4.9:
- Adjust context
- Drop specific part for PUD-sized transparent hugepages. Support
for PUD-sized transparent hugepages was added in v4.11-rc1
]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/huge_memory.c | 19 +++++++------------
1 file changed, 7 insertions(+), 12 deletions(-)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -745,20 +745,15 @@ int vmf_insert_pfn_pmd(struct vm_area_st
EXPORT_SYMBOL_GPL(vmf_insert_pfn_pmd);

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);
}

@@ -787,7 +782,7 @@ struct page *follow_devmap_pmd(struct vm
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
@@ -1158,7 +1153,7 @@ struct page *follow_trans_huge_pmd(struc
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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