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    SubjectRe: [PATCH 4/4] memcg: simplify LRU handling by new rule
    On Wed, Dec 14, 2011 at 04:52:26PM +0900, KAMEZAWA Hiroyuki wrote:
    > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >
    > Now, at LRU handling, memory cgroup needs to do complicated works
    > to see valid pc->mem_cgroup, which may be overwritten.
    >
    > This patch is for relaxing the protocol. This patch guarantees
    > - when pc->mem_cgroup is overwritten, page must not be on LRU.
    >
    > By this, LRU routine can believe pc->mem_cgroup and don't need to
    > check bits on pc->flags. This new rule may adds small overheads to
    > swapin. But in most case, lru handling gets faster.
    >
    > After this patch, PCG_ACCT_LRU bit is obsolete and removed.
    >
    > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >
    > ---
    > include/linux/page_cgroup.h | 8 -----
    > mm/memcontrol.c | 72 ++++++++++--------------------------------
    > 2 files changed, 17 insertions(+), 63 deletions(-)

    This, too, speaks for itself and the logic seems sound to me.

    Acked-by: Johannes Weiner <hannes@cmpxchg.org>

    Minor style things:

    > @@ -974,30 +974,8 @@ struct lruvec *mem_cgroup_lru_add_list(struct zone *zone, struct page *page,
    > return &zone->lruvec;
    >
    > pc = lookup_page_cgroup(page);
    > - VM_BUG_ON(PageCgroupAcctLRU(pc));
    > - /*
    > - * putback: charge:
    > - * SetPageLRU SetPageCgroupUsed
    > - * smp_mb smp_mb
    > - * PageCgroupUsed && add to memcg LRU PageLRU && add to memcg LRU
    > - *
    > - * Ensure that one of the two sides adds the page to the memcg
    > - * LRU during a race.
    > - */
    > - smp_mb();
    > - /*
    > - * If the page is uncharged, it may be freed soon, but it
    > - * could also be swap cache (readahead, swapoff) that needs to
    > - * be reclaimable in the future. root_mem_cgroup will babysit
    > - * it for the time being.
    > - */
    > - if (PageCgroupUsed(pc)) {
    > - /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
    > - smp_rmb();
    > - memcg = pc->mem_cgroup;
    > - SetPageCgroupAcctLRU(pc);
    > - } else
    > - memcg = root_mem_cgroup;
    > + memcg = pc->mem_cgroup;
    > + VM_BUG_ON(!memcg);
    > mz = page_cgroup_zoneinfo(memcg, page);

    I think the memcg local variable is not really needed anymore.

    Also, please don't add bug-ons for simple NULL tests, they are
    redundant when the dereference would blow up just as well.

    > @@ -2399,6 +2368,8 @@ void mem_cgroup_split_huge_fixup(struct page *head)
    > {
    > struct page_cgroup *head_pc = lookup_page_cgroup(head);
    > struct page_cgroup *pc;
    > + struct mem_cgroup_per_zone *mz;
    > + enum lru_list lru;
    > int i;

    You broke the reverse christmas tree sorting!


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