Re: [PATCH 2/2] memcg: fix livelock in try charge during readahead

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

On Mon, 12 Dec 2011 18:16:48 -0800
Ying Han <yinghan@google.com> wrote:

> Couple of kernel dumps are triggered by watchdog timeout. It turns out that two
> processes within a memcg livelock on a same page lock. We believe this is not
> memcg specific issue and the same livelock exists in non-memcg world as well.
> 
> The sequence of triggering the livelock:
> 1. Task_A enters pagefault (filemap_fault) and then starts readahead
> filemap_fault
>  -> do_sync_mmap_readahead
>     -> ra_submit
>        ->__do_page_cache_readahead // here we allocate the readahead pages
>          ->read_pages
>          ...
>            ->add_to_page_cache_locked
>              //for each page, we do the try charge and then add the page into
>              //radix tree. If one of the try charge failed, it enters per-memcg
>              //oom while holding the page lock of previous readahead pages.
> 
>             // in the memcg oom killer, it picks a task within the same memcg
>             // and mark it TIF_MEMDIE. then it goes back into retry loop and
>             // hopes the task exits to free some memory.
> 
> 2. Task_B enters pagefault (filemap_fault) and finds the page in radix tree (
> one of the readahead pages from ProcessA)
> 
> filemap_fault
>  ->__lock_page // here it is marked as TIF_MEMDIE. but it can not proceed since
>                // the page lock is hold by ProcessA looping at OOM.
> 

Should this __lock_page() be lock_page_killable() ?
Hmm, at seeing linux-next, it's now lock_page_or_retry() and FAULT_FLAG_KILLABLE
is set. why not killed immediately ?

Thanks,
-Kame

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