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    Subject[patch 7/9] mm: memcontrol: do not acquire page_cgroup lock for kmem pages
    Date
    Kmem page charging and uncharging is serialized by means of exclusive
    access to the page. Do not take the page_cgroup lock and don't set
    pc->flags atomically.

    Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
    ---
    mm/memcontrol.c | 16 +++-------------
    1 file changed, 3 insertions(+), 13 deletions(-)

    diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    index c528ae9ac230..d3961fce1d54 100644
    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -3535,10 +3535,8 @@ void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
    }

    pc = lookup_page_cgroup(page);
    - lock_page_cgroup(pc);
    pc->mem_cgroup = memcg;
    - SetPageCgroupUsed(pc);
    - unlock_page_cgroup(pc);
    + pc->flags = PCG_USED;
    }

    void __memcg_kmem_uncharge_pages(struct page *page, int order)
    @@ -3548,19 +3546,11 @@ void __memcg_kmem_uncharge_pages(struct page *page, int order)


    pc = lookup_page_cgroup(page);
    - /*
    - * Fast unlocked return. Theoretically might have changed, have to
    - * check again after locking.
    - */
    if (!PageCgroupUsed(pc))
    return;

    - lock_page_cgroup(pc);
    - if (PageCgroupUsed(pc)) {
    - memcg = pc->mem_cgroup;
    - ClearPageCgroupUsed(pc);
    - }
    - unlock_page_cgroup(pc);
    + memcg = pc->mem_cgroup;
    + pc->flags = 0;

    /*
    * We trust that only if there is a memcg associated with the page, it
    --
    1.9.2


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