[Devel] [PATCH RHEL7 COMMIT] mm/ksm: Don't WARN if page is still mapped in remove_stable_node()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-1062.4.2.vz7.116.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1062.4.2.vz7.116.3
------>
commit 9d698729823d0656f5b26ca5ea2ea6f4cdf6be91
Author: Andrey Ryabinin <aryabinin at virtuozzo.com>
Date:   Tue Nov 19 19:11:12 2019 +0300

    mm/ksm: 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.
    
    https://jira.sw.ru/browse/PSBM-99301
    
    Fixes: cbf86cfe04a6 ("ksm: remove old stable nodes more thoroughly")
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/ksm.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 650dfb20b7b9..a8422a8862c1 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -846,13 +846,13 @@ static int remove_stable_node(struct stable_node *stable_node)
 		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