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    Subject[PATCH 3.10 310/319] mm,ksm: fix endless looping in allocating memory when ksm enable
    Date
    From: zhong jiang <zhongjiang@huawei.com>

    commit 5b398e416e880159fe55eefd93c6588fa072cd66 upstream.

    I hit the following hung task when runing a OOM LTP test case with 4.1
    kernel.

    Call trace:
    [<ffffffc000086a88>] __switch_to+0x74/0x8c
    [<ffffffc000a1bae0>] __schedule+0x23c/0x7bc
    [<ffffffc000a1c09c>] schedule+0x3c/0x94
    [<ffffffc000a1eb84>] rwsem_down_write_failed+0x214/0x350
    [<ffffffc000a1e32c>] down_write+0x64/0x80
    [<ffffffc00021f794>] __ksm_exit+0x90/0x19c
    [<ffffffc0000be650>] mmput+0x118/0x11c
    [<ffffffc0000c3ec4>] do_exit+0x2dc/0xa74
    [<ffffffc0000c46f8>] do_group_exit+0x4c/0xe4
    [<ffffffc0000d0f34>] get_signal+0x444/0x5e0
    [<ffffffc000089fcc>] do_signal+0x1d8/0x450
    [<ffffffc00008a35c>] do_notify_resume+0x70/0x78

    The oom victim cannot terminate because it needs to take mmap_sem for
    write while the lock is held by ksmd for read which loops in the page
    allocator

    ksm_do_scan
    scan_get_next_rmap_item
    down_read
    get_next_rmap_item
    alloc_rmap_item #ksmd will loop permanently.

    There is no way forward because the oom victim cannot release any memory
    in 4.1 based kernel. Since 4.6 we have the oom reaper which would solve
    this problem because it would release the memory asynchronously.
    Nevertheless we can relax alloc_rmap_item requirements and use
    __GFP_NORETRY because the allocation failure is acceptable as ksm_do_scan
    would just retry later after the lock got dropped.

    Such a patch would be also easy to backport to older stable kernels which
    do not have oom_reaper.

    While we are at it add GFP_NOWARN so the admin doesn't have to be alarmed
    by the allocation failure.

    Link: http://lkml.kernel.org/r/1474165570-44398-1-git-send-email-zhongjiang@huawei.com
    Signed-off-by: zhong jiang <zhongjiang@huawei.com>
    Suggested-by: Hugh Dickins <hughd@google.com>
    Suggested-by: Michal Hocko <mhocko@suse.cz>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Acked-by: Hugh Dickins <hughd@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    ---
    mm/ksm.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/mm/ksm.c b/mm/ksm.c
    index 7bf748f..d1b19b9 100644
    --- a/mm/ksm.c
    +++ b/mm/ksm.c
    @@ -283,7 +283,8 @@ static inline struct rmap_item *alloc_rmap_item(void)
    {
    struct rmap_item *rmap_item;

    - rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
    + rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL |
    + __GFP_NORETRY | __GFP_NOWARN);
    if (rmap_item)
    ksm_rmap_items++;
    return rmap_item;
    --
    2.8.0.rc2.1.gbe9624a
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