[RFC] mm: make every pte dirty on do_swap_page

[Minchan Kim]

Bascially, MADV_FREE relys on the pte dirty to decide whether
VM should discard or not. However, swapped-in page doesn't have
pte_dirty. Instead, it checks PageDirty and PageSwapCache for
such page because swapped-in page could live on swap cache or
set PageDirty when it is removed from swapcache so MADV_FREE
checks it and doesn't discard.

The problem in here is any anonymous page can have PageDirty if
it is removed from swapcache so that VM cannot parse those pages
as freeable even if we did madvise_free. Look at below example.

ptr = malloc();
memset(ptr);
..
heavy memory pressure -> swap-out all of pages
..
out of memory pressure
..
var = *ptr; -> swap-in page/remove the page from swapcache. so pte_clean
               but SetPageDirty

madvise_free(ptr);
..
..
heavy memory pressure -> VM cannot discard the page by PageDirty.

PageDirty for anonymous page aims for avoiding duplicating
swapping out. In other words, if a page have swapped-in but
live swapcache(ie, !PageDirty), we could save swapout if the page
is selected as victim by VM in future because swap device have
kept previous swapped-out contents of the page.

So, rather than relying on the PG_dirty for working madvise_free,
pte_dirty is more straightforward.
Inherently, swapped-out page was pte_dirty so this patch restores
the dirtiness when swap-in fault happens and madvise_free doesn't
rely on the PageDirty.

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/madvise.c | 1 -
 mm/memory.c  | 9 +++++++--
 mm/rmap.c    | 2 +-
 mm/vmscan.c  | 3 +--
 4 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 6d0fcb8..d64200e 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -309,7 +309,6 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 				continue;
 			}
 
-			ClearPageDirty(page);
 			unlock_page(page);
 		}
 
diff --git a/mm/memory.c b/mm/memory.c
index 8ae52c9..2f45e77 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2460,9 +2460,14 @@ 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);
+
+	/*
+	 * Every page swapped-out was pte_dirty so we makes pte dirty again.
+	 * MADV_FREE relys on it.
+	 */
+	pte = mk_pte(pte_mkdirty(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;
diff --git a/mm/rmap.c b/mm/rmap.c
index 47b3ba8..34c1d66 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1268,7 +1268,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 
 		if (flags & TTU_FREE) {
 			VM_BUG_ON_PAGE(PageSwapCache(page), page);
-			if (!dirty && !PageDirty(page)) {
+			if (!dirty) {
 				/* It's a freeable page by MADV_FREE */
 				dec_mm_counter(mm, MM_ANONPAGES);
 				goto discard;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 671e47e..7f520c9 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -805,8 +805,7 @@ static enum page_references page_check_references(struct page *page,
 		return PAGEREF_KEEP;
 	}
 
-	if (PageAnon(page) && !pte_dirty && !PageSwapCache(page) &&
-			!PageDirty(page))
+	if (PageAnon(page) && !pte_dirty && !PageSwapCache(page))
 		*freeable = true;
 
 	/* Reclaim if clean, defer dirty pages to writeback */
-- 
1.9.3

-- 
Kind regards,
Minchan Kim
--
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