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

[Minchan Kim <minchan@kernel.org>]

On Wed, Dec 14, 2011 at 3:29 AM, Ying Han <yinghan@google.com> wrote:
> On Mon, Dec 12, 2011 at 10:10 PM, Minchan Kim <minchan@kernel.org> wrote:
>> On Mon, Dec 12, 2011 at 06:16:48PM -0800, Ying Han 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.
>>>
>>> Since the TIF_MEMDIE task_B is live locked, it ends up blocking other tasks
>>> making forward progress since they are also checking the flag in
>>> select_bad_process. The same issue exists in the non-memcg world. Instead of
>>> entering oom through mem_cgroup_cache_charge(), we might enter it through
>>> radix_tree_preload().
>>>
>>> The proposed fix here is to pass __GFP_NORETRY gfp_mask into try charge under
>>> readahead. Then we skip entering memcg OOM kill which eliminates the case where
>>> it OOMs on one page and holds other page locks. It seems to be safe to do that
>>> since both filemap_fault() and do_generic_file_read() handles the fallback case
>>> of "no_cached_page".
>>>
>>> Note:
>>> After this patch, we might experience some charge fails for readahead pages
>>> (since we don't enter oom). But this sounds sane compared to letting the system
>>> trying extremely hard to charge a readahead page by doing reclaim and then oom,
>>> the later one also triggers livelock as listed above.
>>>
>>> Signed-off-by: Greg Thelen <gthelen@google.com>
>>> Signed-off-by: Ying Han <yinghan@google.com>
>>
>> Nice catch.
>>
>> The concern is GFP_KERNEL != avoid OOM.
>> Although it works now, it can be changed.
>>
>> With alternative idea, We can use explicit oom_killer_disable with __GFP_NOWARN
>> but it wouldn't work since oom_killer_disabled isn't reference count variable.
>> Of course, we can change it with reference-counted atomic variable.
>> The benefit is it's more explicit and doesn't depends on __GFP_NORETRY implementation.
>> So I don't have a good idea except above.
>
>> If you want __GFP_NORTRY patch, thing we can do best is add comment in detail, at least.
>> both side, here add_to_page_cache_lru and there __GFP_NORETRY in include/linux/gfp.h.
>
> Correct me in case i missed something, looks like I want to backport
> the " x86,mm: make pagefault killable" patch, and we might be able to
> solve the livelock w/o changing the readahead code.
>

I missed lock_page_or_retry Kame pointed out.
So, backport should solve the problem.

-- 
Kind regards,
Minchan Kim

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