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    Subject[patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns NULL
    Date
    From: Johannes Weiner <jweiner@redhat.com>

    Pages have their corresponding page_cgroup descriptors set up before
    they are used in userspace, and thus managed by a memory cgroup.

    The only time where lookup_page_cgroup() can return NULL is in the
    page sanity checking code that executes while feeding pages into the
    page allocator for the first time.

    Remove the NULL checks against lookup_page_cgroup() results from all
    callsites where we know that corresponding page_cgroup descriptors
    must be allocated.

    Signed-off-by: Johannes Weiner <jweiner@redhat.com>
    ---
    mm/memcontrol.c | 8 ++------
    1 files changed, 2 insertions(+), 6 deletions(-)
    diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    index d825af9..d4d139a 100644
    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -1894,9 +1894,6 @@ void mem_cgroup_update_page_stat(struct page *page,
    bool need_unlock = false;
    unsigned long uninitialized_var(flags);

    - if (unlikely(!pc))
    - return;
    -
    rcu_read_lock();
    memcg = pc->mem_cgroup;
    if (unlikely(!memcg || !PageCgroupUsed(pc)))
    @@ -2669,8 +2666,6 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
    }

    pc = lookup_page_cgroup(page);
    - BUG_ON(!pc); /* XXX: remove this and move pc lookup into commit */
    -
    ret = __mem_cgroup_try_charge(mm, gfp_mask, nr_pages, &memcg, oom);
    if (ret || !memcg)
    return ret;
    @@ -2942,7 +2937,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
    * Check if our page_cgroup is valid
    */
    pc = lookup_page_cgroup(page);
    - if (unlikely(!pc || !PageCgroupUsed(pc)))
    + if (unlikely(!PageCgroupUsed(pc)))
    return NULL;

    lock_page_cgroup(pc);
    @@ -3326,6 +3321,7 @@ static struct page_cgroup *lookup_page_cgroup_used(struct page *page)
    struct page_cgroup *pc;

    pc = lookup_page_cgroup(page);
    + /* Can be NULL while bootstrapping the page allocator */
    if (likely(pc) && PageCgroupUsed(pc))
    return pc;
    return NULL;
    --
    1.7.6.4


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