[RFC 3/6] mm: mark dirty bit on swapped-in page

[Minchan Kim]

Basically, MADV_FREE relys on the dirty bit in page table entry
to decide whether VM allows to discard the page or not.
IOW, if page table entry includes marked dirty bit, VM shouldn't
discard the page.

However, if swap-in by read fault happens, page table entry
point out the page doesn't have marked dirty bit so MADV_FREE
might discard the page wrongly.

To fix the problem, this patch marks page table entry of page
swapping-in as dirty so VM shouldn't discard the page suddenly
under us.

With MADV_FREE point of view, marking dirty unconditionally is
no problem because we dropped swapped page in MADV_FREE sycall
context(ie, Look at madvise_free_pte_range) so every swapping-in
pages are no MADV_FREE hinted pages.

Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
Reported-by: Yalin Wang <yalin.wang@xxxxxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/memory.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 8a2fc9945b46..d1709f763152 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2557,9 +2557,11 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	inc_mm_counter_fast(mm, MM_ANONPAGES);
 	dec_mm_counter_fast(mm, MM_SWAPENTS);
-	pte = mk_pte(page, vma->vm_page_prot);
+
+	/* Mark dirty bit of page table because MADV_FREE relies on it */
+	pte = pte_mkdirty(mk_pte(page, vma->vm_page_prot));
 	if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
-		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
+		pte = maybe_mkwrite(pte, vma);
 		flags &= ~FAULT_FLAG_WRITE;
 		ret |= VM_FAULT_WRITE;
 		exclusive = 1;
-- 
1.9.1

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