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    Subject[PATCH 5.9 251/252] mm: memcg/slab: fix root memcg vmstats
    Date
    From: Muchun Song <songmuchun@bytedance.com>

    commit 8faeb1ffd79593c9cd8a2a80ecdda371e3b826cb upstream.

    If we reparent the slab objects to the root memcg, when we free the slab
    object, we need to update the per-memcg vmstats to keep it correct for
    the root memcg. Now this at least affects the vmstat of
    NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack size is
    smaller than the PAGE_SIZE.

    David said:
    "I assume that without this fix that the root memcg's vmstat would
    always be inflated if we reparented"

    Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
    Signed-off-by: Muchun Song <songmuchun@bytedance.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Reviewed-by: Shakeel Butt <shakeelb@google.com>
    Acked-by: Roman Gushchin <guro@fb.com>
    Acked-by: Johannes Weiner <hannes@cmpxchg.org>
    Acked-by: David Rientjes <rientjes@google.com>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
    Cc: Christopher Lameter <cl@linux.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Roman Gushchin <guro@fb.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Yafang Shao <laoar.shao@gmail.com>
    Cc: Chris Down <chris@chrisdown.name>
    Cc: <stable@vger.kernel.org> [5.3+]
    Link: https://lkml.kernel.org/r/20201110031015.15715-1-songmuchun@bytedance.com
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/memcontrol.c | 9 +++++++--
    1 file changed, 7 insertions(+), 2 deletions(-)

    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -872,8 +872,13 @@ void __mod_lruvec_slab_state(void *p, en
    rcu_read_lock();
    memcg = mem_cgroup_from_obj(p);

    - /* Untracked pages have no memcg, no lruvec. Update only the node */
    - if (!memcg || memcg == root_mem_cgroup) {
    + /*
    + * Untracked pages have no memcg, no lruvec. Update only the
    + * node. If we reparent the slab objects to the root memcg,
    + * when we free the slab object, we need to update the per-memcg
    + * vmstats to keep it correct for the root memcg.
    + */
    + if (!memcg) {
    __mod_node_page_state(pgdat, idx, val);
    } else {
    lruvec = mem_cgroup_lruvec(memcg, pgdat);

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