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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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