Re: [PATCH 2/4] memcg: simplify corner case handling of LRU.

Michal Hocko
2011-12-20T15:05:33Z

On Wed 14-12-11 16:50:32, KAMEZAWA Hiroyuki wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> This patch simplifies LRU handling of racy case (memcg+SwapCache).
> At charging, SwapCache tend to be on LRU already. So, before
> overwriting pc->mem_cgroup, the page must be removed from LRU and
> added to LRU later.
>
> This patch does
> spin_lock(zone->lru_lock);
> if (PageLRU(page))
> remove from LRU
> overwrite pc->mem_cgroup
> if (PageLRU(page))
> add to new LRU.
> spin_unlock(zone->lru_lock);
>
> And guarantee all pages are not on LRU at modifying pc->mem_cgroup.
> This patch also unfies lru handling of replace_page_cache() and
> swapin.
>
> Changelog:
> - modify PageLRU flag correctly.
> - handle replace_page_cache.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> mm/memcontrol.c | 109 ++++++++-----------------------------------------------
> 1 files changed, 16 insertions(+), 93 deletions(-)

Wow, really nice. I always hated {before,after}_commit thingies. It was
just too complex.

After ClearPageLRU && SetPageLRU cleanup mentioned by Johannes already
feel free to add my
Acked-by: Michal Hocko <mhocko@suse.cz>

>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 947c62c..7a857e8 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1071,86 +1071,6 @@ struct lruvec *mem_cgroup_lru_move_lists(struct zone *zone,
> }
>
> /*
> - * At handling SwapCache and other FUSE stuff, pc->mem_cgroup may be changed
> - * while it's linked to lru because the page may be reused after it's fully
> - * uncharged. To handle that, unlink page_cgroup from LRU when charge it again.
> - * It's done under lock_page and expected that zone->lru_lock isnever held.
> - */
> -static void mem_cgroup_lru_del_before_commit(struct page *page)
> -{
> - enum lru_list lru;
> - unsigned long flags;
> - struct zone *zone = page_zone(page);
> - struct page_cgroup *pc = lookup_page_cgroup(page);
> -
> - /*
> - * Doing this check without taking ->lru_lock seems wrong but this
> - * is safe. Because if page_cgroup's USED bit is unset, the page
> - * will not be added to any memcg's LRU. If page_cgroup's USED bit is
> - * set, the commit after this will fail, anyway.
> - * This all charge/uncharge is done under some mutual execustion.
> - * So, we don't need to taking care of changes in USED bit.
> - */
> - if (likely(!PageLRU(page)))
> - return;
> -
> - spin_lock_irqsave(&zone->lru_lock, flags);
> - lru = page_lru(page);
> - /*
> - * The uncharged page could still be registered to the LRU of
> - * the stale pc->mem_cgroup.
> - *
> - * As pc->mem_cgroup is about to get overwritten, the old LRU
> - * accounting needs to be taken care of. Let root_mem_cgroup
> - * babysit the page until the new memcg is responsible for it.
> - *
> - * The PCG_USED bit is guarded by lock_page() as the page is
> - * swapcache/pagecache.
> - */
> - if (PageLRU(page) && PageCgroupAcctLRU(pc) && !PageCgroupUsed(pc)) {
> - del_page_from_lru_list(zone, page, lru);
> - add_page_to_lru_list(zone, page, lru);
> - }
> - spin_unlock_irqrestore(&zone->lru_lock, flags);
> -}
> -
> -static void mem_cgroup_lru_add_after_commit(struct page *page)
> -{
> - enum lru_list lru;
> - unsigned long flags;
> - struct zone *zone = page_zone(page);
> - struct page_cgroup *pc = lookup_page_cgroup(page);
> - /*
> - * 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();
> - /* taking care of that the page is added to LRU while we commit it */
> - if (likely(!PageLRU(page)))
> - return;
> - spin_lock_irqsave(&zone->lru_lock, flags);
> - lru = page_lru(page);
> - /*
> - * If the page is not on the LRU, someone will soon put it
> - * there. If it is, and also already accounted for on the
> - * memcg-side, it must be on the right lruvec as setting
> - * pc->mem_cgroup and PageCgroupUsed is properly ordered.
> - * Otherwise, root_mem_cgroup has been babysitting the page
> - * during the charge. Move it to the new memcg now.
> - */
> - if (PageLRU(page) && !PageCgroupAcctLRU(pc)) {
> - del_page_from_lru_list(zone, page, lru);
> - add_page_to_lru_list(zone, page, lru);
> - }
> - spin_unlock_irqrestore(&zone->lru_lock, flags);
> -}
> -
> -/*
> * Checks whether given mem is same or in the root_mem_cgroup's
> * hierarchy subtree
> */
> @@ -2695,14 +2615,27 @@ __mem_cgroup_commit_charge_lrucare(struct page *page, struct mem_cgroup *memcg,
> enum charge_type ctype)
> {
> struct page_cgroup *pc = lookup_page_cgroup(page);
> + struct zone *zone = page_zone(page);
> + unsigned long flags;
> + bool removed = false;
> +
> /*
> * In some case, SwapCache, FUSE(splice_buf->radixtree), the page
> * is already on LRU. It means the page may on some other page_cgroup's
> * LRU. Take care of it.
> */
> - mem_cgroup_lru_del_before_commit(page);
> + spin_lock_irqsave(&zone->lru_lock, flags);
> + if (PageLRU(page)) {
> + del_page_from_lru_list(zone, page, page_lru(page));
> + ClearPageLRU(page);
> + removed = true;
> + }
> __mem_cgroup_commit_charge(memcg, page, 1, pc, ctype);
> - mem_cgroup_lru_add_after_commit(page);
> + if (removed) {
> + add_page_to_lru_list(zone, page, page_lru(page));
> + SetPageLRU(page);
> + }
> + spin_unlock_irqrestore(&zone->lru_lock, flags);
> return;
> }
>
> @@ -3303,9 +3236,7 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,

> {
> struct mem_cgroup *memcg;
> struct page_cgroup *pc;
> - struct zone *zone;
> enum charge_type type = MEM_CGROUP_CHARGE_TYPE_CACHE;
> - unsigned long flags;
>
> pc = lookup_page_cgroup(oldpage);
> /* fix accounting on old pages */
> @@ -3318,20 +3249,12 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,
> if (PageSwapBacked(oldpage))
> type = MEM_CGROUP_CHARGE_TYPE_SHMEM;
>
> - zone = page_zone(newpage);
> - pc = lookup_page_cgroup(newpage);
> /*
> * Even if newpage->mapping was NULL before starting replacement,
> * the newpage may be on LRU(or pagevec for LRU) already. We lock
> * LRU while we overwrite pc->mem_cgroup.
> */
> - spin_lock_irqsave(&zone->lru_lock, flags);
> - if (PageLRU(newpage))
> - del_page_from_lru_list(zone, newpage, page_lru(newpage));
> - __mem_cgroup_commit_charge(memcg, newpage, 1, pc, type);
> - if (PageLRU(newpage))
> - add_page_to_lru_list(zone, newpage, page_lru(newpage));
> - spin_unlock_irqrestore(&zone->lru_lock, flags);
> + __mem_cgroup_commit_charge_lrucare(newpage, memcg, type);
> }
>
> #ifdef CONFIG_DEBUG_VM
> --
> 1.7.4.1
>
>
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--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic