[PATCH 3.16 135/136] mm/ksm.c: don't WARN if page is still mapped in remove_stable_node()

[Ben Hutchings]

3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>

commit 9a63236f1ad82d71a98aa80320b6cb618fb32f44 upstream.

It's possible to hit the WARN_ON_ONCE(page_mapped(page)) in
remove_stable_node() when it races with __mmput() and squeezes in
between ksm_exit() and exit_mmap().

  WARNING: CPU: 0 PID: 3295 at mm/ksm.c:888 remove_stable_node+0x10c/0x150

  Call Trace:
   remove_all_stable_nodes+0x12b/0x330
   run_store+0x4ef/0x7b0
   kernfs_fop_write+0x200/0x420
   vfs_write+0x154/0x450
   ksys_write+0xf9/0x1d0
   do_syscall_64+0x99/0x510
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Remove the warning as there is nothing scary going on.

Link: http://lkml.kernel.org/r/20191119131850.5675-1-aryabinin@xxxxxxxxxxxxx
Fixes: cbf86cfe04a6 ("ksm: remove old stable nodes more thoroughly")
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/ksm.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -715,13 +715,13 @@ static int remove_stable_node(struct sta
 		return 0;
 	}
 
-	if (WARN_ON_ONCE(page_mapped(page))) {
-		/*
-		 * This should not happen: but if it does, just refuse to let
-		 * merge_across_nodes be switched - there is no need to panic.
-		 */
-		err = -EBUSY;
-	} else {
+	/*
+	 * Page could be still mapped if this races with __mmput() running in
+	 * between ksm_exit() and exit_mmap(). Just refuse to let
+	 * merge_across_nodes/max_page_sharing be switched.
+	 */
+	err = -EBUSY;
+	if (!page_mapped(page)) {
 		/*
 		 * The stable node did not yet appear stale to get_ksm_page(),
 		 * since that allows for an unmapped ksm page to be recognized