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    Subject[PATCH 4.9 009/151] mm/ksm.c: dont WARN if page is still mapped in remove_stable_node()
    Date
    From: Andrey Ryabinin <aryabinin@virtuozzo.com>

    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@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>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/mm/ksm.c
    +++ b/mm/ksm.c
    @@ -710,13 +710,13 @@ 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

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