[patch 4/4] mm/ksm.c: don't WARN if page is still mapped in remove_stable_node()

[akpm@linux-foundation.org]

From: Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: mm/ksm.c: don't WARN if page is still mapped in remove_stable_node()

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@virtuozzo.com
Fixes: cbf86cfe04a6 ("ksm: remove old stable nodes more thoroughly")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/ksm.c |   14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

--- a/mm/ksm.c~mm-ksm-dont-warn-if-page-is-still-mapped-in-remove_stable_node
+++ a/mm/ksm.c
@@ -885,13 +885,13 @@ 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
_