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    Subject[PATCH 5.2 100/131] mm: vmscan: check if mem cgroup is disabled or not before calling memcg slab shrinker
    Date
    From: Yang Shi <yang.shi@linux.alibaba.com>

    commit fa1e512fac717f34e7c12d7a384c46e90a647392 upstream.

    Shakeel Butt reported premature oom on kernel with
    "cgroup_disable=memory" since mem_cgroup_is_root() returns false even
    though memcg is actually NULL. The drop_caches is also broken.

    It is because commit aeed1d325d42 ("mm/vmscan.c: generalize
    shrink_slab() calls in shrink_node()") removed the !memcg check before
    !mem_cgroup_is_root(). And, surprisingly root memcg is allocated even
    though memory cgroup is disabled by kernel boot parameter.

    Add mem_cgroup_disabled() check to make reclaimer work as expected.

    Link: http://lkml.kernel.org/r/1563385526-20805-1-git-send-email-yang.shi@linux.alibaba.com
    Fixes: aeed1d325d42 ("mm/vmscan.c: generalize shrink_slab() calls in shrink_node()")
    Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
    Reported-by: Shakeel Butt <shakeelb@google.com>
    Reviewed-by: Shakeel Butt <shakeelb@google.com>
    Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Cc: Jan Hadrava <had@kam.mff.cuni.cz>
    Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Roman Gushchin <guro@fb.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Qian Cai <cai@lca.pw>
    Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: <stable@vger.kernel.org> [4.19+]
    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/vmscan.c | 9 ++++++++-
    1 file changed, 8 insertions(+), 1 deletion(-)

    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -684,7 +684,14 @@ static unsigned long shrink_slab(gfp_t g
    unsigned long ret, freed = 0;
    struct shrinker *shrinker;

    - if (!mem_cgroup_is_root(memcg))
    + /*
    + * The root memcg might be allocated even though memcg is disabled
    + * via "cgroup_disable=memory" boot parameter. This could make
    + * mem_cgroup_is_root() return false, then just run memcg slab
    + * shrink, but skip global shrink. This may result in premature
    + * oom.
    + */
    + if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
    return shrink_slab_memcg(gfp_mask, nid, memcg, priority);

    if (!down_read_trylock(&shrinker_rwsem))

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