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    SubjectRe: [PATCH 5/6] memcg: lazy lru freeing
    On Wed, 1 Oct 2008 17:00:05 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > Free page_cgroup from its LRU in batched manner.
    >
    > When uncharge() is called, page is pushed onto per-cpu vector and
    > removed from LRU, later.. This routine resembles to global LRU's pagevec.
    > This patch is half of the whole patch and a set with following lazy LRU add
    > patch.
    >
    > After this, a pc, which is PageCgroupLRU(pc)==true, is on LRU.
    > This LRU bit is guarded by lru_lock().
    >
    > PageCgroupUsed(pc) && PageCgroupLRU(pc) means "pc" is used and on LRU.
    > This check makes sense only when both 2 locks, lock_page_cgroup()/lru_lock(),
    > are aquired.
    >
    > PageCgroupUsed(pc) && !PageCgroupLRU(pc) means "pc" is used but not on LRU.
    > !PageCgroupUsed(pc) && PageCgroupLRU(pc) means "pc" is unused but still on
    > LRU. lru walk routine should avoid touching this.
    >
    > Changelog (v5) => (v6):
    > - Fixing race and added PCG_LRU bit
    >
    > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >

    (snip)

    > +static void
    > +__release_page_cgroup(struct memcg_percpu_vec *mpv)
    > +{
    > + unsigned long flags;
    > + struct mem_cgroup_per_zone *mz, *prev_mz;
    > + struct page_cgroup *pc;
    > + int i, nr;
    > +
    > + local_irq_save(flags);
    > + nr = mpv->nr;
    > + mpv->nr = 0;
    > + prev_mz = NULL;
    > + for (i = nr - 1; i >= 0; i--) {
    > + pc = mpv->vec[i];
    > + mz = page_cgroup_zoneinfo(pc);
    > + if (prev_mz != mz) {
    > + if (prev_mz)
    > + spin_unlock(&prev_mz->lru_lock);
    > + prev_mz = mz;
    > + spin_lock(&mz->lru_lock);
    > + }
    > + /*
    > + * this "pc" may be charge()->uncharge() while we are waiting
    > + * for this. But charge() path check LRU bit and remove this
    > + * from LRU if necessary.
    > + */
    > + if (!PageCgroupUsed(pc) && PageCgroupLRU(pc)) {
    > + ClearPageCgroupLRU(pc);
    > + __mem_cgroup_remove_list(mz, pc);
    > + css_put(&pc->mem_cgroup->css);
    > + }
    > + }
    > + if (prev_mz)
    > + spin_unlock(&prev_mz->lru_lock);
    > + local_irq_restore(flags);
    > +
    > +}
    > +
    I'm wondering if page_cgroup_zoneinfo is safe without lock_page_cgroup
    because it dereferences pc->mem_cgroup.
    I'm worring if the pc has been moved to another lru by re-charge(and re-uncharge),
    and __mem_cgroup_remove_list toches a wrong(old) group.

    Hmm, there are many things to be done for re-charge and re-uncharge,
    so "if (!PageCgroupUsed(pc) && PageCgroupLRU(pc))" would be enough.
    (it can avoid race between re-charge.)

    Another user of page_cgroup_zoneinfo without lock_page_cgroup is
    __mem_cgroup_move_lists called by mem_cgroup_isolate_pages,
    but mem_cgroup_isolate_pages handles pc which is actually on the mz->lru
    so it would be ok.
    (I think adding VM_BUG_ON(mz != page_cgroup_zoneifno(pc)) would make sense,
    or add new arg *mz to __mem_cgroup_move_lists?)


    Thanks,
    Daisuke Nishimura.


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