Re: [PATCH] fix unused/stale swap cache handling on memcg v3

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 21 Apr 2009 11:35:25 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> @@ -785,6 +786,23 @@ activate_locked:
>                 SetPageActive(page);
>                 pgactivate++;
>  keep_locked:
> +               if (!scanning_global_lru(sc) && PageSwapCache(page)) {
> +                       struct page_cgroup *pc;
> +
> +                       pc = lookup_page_cgroup(page);
> +                       /*
> +                        * Used bit of swapcache is solid under page lock.
> +                        */
> +                       if (unlikely(!PageCgroupUsed(pc)))
> +                               /*
> +                                * This can happen if the page is free'ed by
> +                                * the owner process before it is added to
> +                                * swapcache.
> +                                * These swapcache cannot be managed by memcg
> +                                * well, so free it here.
> +                                */
> +                               try_to_free_swap(page);
> +               }
>                 unlock_page(page);
>  keep:
>                 list_add(&page->lru, &ret_pages);
> 
> This cannot prevent type-1 orphan SwapCache(caused by the race
> between exit() and swap-in readahead).
> Type-1 can pressure the memsw usage(trigger OOM if memsw.limit is set, as a result)
> and make struct mem_cgroup unfreeable even after rmdir(because it holds refcount
> to mem_cgroup).
Hmm.
   free_swap_cache()
	-> trylock_page() => failure case ?

add following codes.
==
 588                         page = find_get_page(&swapper_space, entry.val);
 589                         if (page && !trylock_page(page)) {
				     mem_cgroup_retry_free_swap_lazy(page);  <=====
 590                                 page_cache_release(page);
 591                                 page = NULL;
 592                         }
==
and  do some kind of lazy ops..I'll try some.

> 
> Do you have any ideas to solve orphan SwapCache problem by adding some hooks to shrink_zone() ?
> (scan some pages from global LRU and check whether it's orphan SwapCache or not by
> adding some code like above ?)
> 
> And, what do you think about adding above code to shrink_page_list() ?
> I think it might be unnecessary if we can solve the problem in another way, though.
> 

I think your hook itself is not very bad. (even if we remove this later..)

Thanks,
-Kame

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