[PATCHv2 2/3] mm: Do not loose dirty and access bits in pmdp_invalidate()

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Vlastimil noted that pmdp_invalidate() is not atomic and we can loose
dirty and access bits if CPU sets them after pmdp dereference, but
before set_pmd_at().

The bug doesn't lead to user-visible misbehaviour in current kernel.

Loosing access bit can lead to sub-optimal reclaim behaviour for THP,
but nothing destructive.

Loosing dirty bit is not a big deal too: we would make page dirty
unconditionally on splitting huge page.

The fix is critical for future work on THP: both huge-ext4 and THP swap
out rely on proper dirty tracking.

The patch change pmdp_invalidate() to make the entry non-present atomically and
return previous value of the entry. This value can be used to check if
CPU set dirty/accessed bits under us.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Vlastimil Babka <vbabka@suse.cz>
---
 include/asm-generic/pgtable.h | 2 +-
 mm/pgtable-generic.c          | 9 +++++----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 7dfa767dc680..ece5e399567a 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -309,7 +309,7 @@ extern pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp);
 #endif
 
 #ifndef __HAVE_ARCH_PMDP_INVALIDATE
-extern void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
+extern pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
 			    pmd_t *pmdp);
 #endif
 
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index c99d9512a45b..148fe36f61a7 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -179,12 +179,13 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp)
 #endif
 
 #ifndef __HAVE_ARCH_PMDP_INVALIDATE
-void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
+pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
 		     pmd_t *pmdp)
 {
-	pmd_t entry = *pmdp;
-	set_pmd_at(vma->vm_mm, address, pmdp, pmd_mknotpresent(entry));
-	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
+	pmd_t old = pmdp_establish(pmdp, pmd_mknotpresent(*pmdp));
+	if (pmd_present(old))
+		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
+	return old;
 }
 #endif
 
-- 
2.11.0

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