[3.11.y.z extended stable] Patch "mm: try_to_unmap_cluster() should lock_page() before mlocking" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    mm: try_to_unmap_cluster() should lock_page() before mlocking

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 8726be29b780103dcbed8350612ebad0fdbd4c71 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka at suse.cz>
Date: Mon, 7 Apr 2014 15:37:50 -0700
Subject: mm: try_to_unmap_cluster() should lock_page() before mlocking

commit 57e68e9cd65b4b8eb4045a1e0d0746458502554c upstream.

A BUG_ON(!PageLocked) was triggered in mlock_vma_page() by Sasha Levin
fuzzing with trinity.  The call site try_to_unmap_cluster() does not lock
the pages other than its check_page parameter (which is already locked).

The BUG_ON in mlock_vma_page() is not documented and its purpose is
somewhat unclear, but apparently it serializes against page migration,
which could otherwise fail to transfer the PG_mlocked flag.  This would
not be fatal, as the page would be eventually encountered again, but
NR_MLOCK accounting would become distorted nevertheless.  This patch adds
a comment to the BUG_ON in mlock_vma_page() and munlock_vma_page() to that
effect.

The call site try_to_unmap_cluster() is fixed so that for page !=
check_page, trylock_page() is attempted (to avoid possible deadlocks as we
already have check_page locked) and mlock_vma_page() is performed only
upon success.  If the page lock cannot be obtained, the page is left
without PG_mlocked, which is again not a problem in the whole unevictable
memory design.

Signed-off-by: Vlastimil Babka <vbabka at suse.cz>
Signed-off-by: Bob Liu <bob.liu at oracle.com>
Reported-by: Sasha Levin <sasha.levin at oracle.com>
Cc: Wanpeng Li <liwanp at linux.vnet.ibm.com>
Cc: Michel Lespinasse <walken at google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
Acked-by: Rik van Riel <riel at redhat.com>
Cc: David Rientjes <rientjes at google.com>
Cc: Mel Gorman <mgorman at suse.de>
Cc: Hugh Dickins <hughd at google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim at lge.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 mm/mlock.c |  2 ++
 mm/rmap.c  | 14 ++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 79b7cf7..713e462 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -76,6 +76,7 @@ void clear_page_mlock(struct page *page)
  */
 void mlock_vma_page(struct page *page)
 {
+	/* Serialize with page migration */
 	BUG_ON(!PageLocked(page));

 	if (!TestSetPageMlocked(page)) {
@@ -106,6 +107,7 @@ unsigned int munlock_vma_page(struct page *page)
 {
 	unsigned int page_mask = 0;

+	/* For try_to_munlock() and to serialize with page migration */
 	BUG_ON(!PageLocked(page));

 	if (TestClearPageMlocked(page)) {
diff --git a/mm/rmap.c b/mm/rmap.c
index 75f4c82..54935c5 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1395,9 +1395,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
 		BUG_ON(!page || PageAnon(page));

 		if (locked_vma) {
-			mlock_vma_page(page);   /* no-op if already mlocked */
-			if (page == check_page)
+			if (page == check_page) {
+				/* we know we have check_page locked */
+				mlock_vma_page(page);
 				ret = SWAP_MLOCK;
+			} else if (trylock_page(page)) {
+				/*
+				 * If we can lock the page, perform mlock.
+				 * Otherwise leave the page alone, it will be
+				 * eventually encountered again later.
+				 */
+				mlock_vma_page(page);
+				unlock_page(page);
+			}
 			continue;	/* don't unmap */
 		}

--
1.9.1