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

[Ying Han <yinghan@google.com>]

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.

Thanks

--Ying

>
>> ---
>>  fs/mpage.c     |    3 ++-
>>  mm/readahead.c |    3 ++-
>>  2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/mpage.c b/fs/mpage.c
>> index 643e9f5..90d608e 100644
>> --- a/fs/mpage.c
>> +++ b/fs/mpage.c
>> @@ -380,7 +380,8 @@ mpage_readpages(struct address_space *mapping, struct list_head *pages,
>>               prefetchw(&page->flags);
>>               list_del(&page->lru);
>>               if (!add_to_page_cache_lru(page, mapping,
>> -                                     page->index, GFP_KERNEL)) {
>> +                                     page->index,
>> +                                     GFP_KERNEL | __GFP_NORETRY)) {
>>                       bio = do_mpage_readpage(bio, page,
>>                                       nr_pages - page_idx,
>>                                       &last_block_in_bio, &map_bh,
>> diff --git a/mm/readahead.c b/mm/readahead.c
>> index cbcbb02..bc9431c 100644
>> --- a/mm/readahead.c
>> +++ b/mm/readahead.c
>> @@ -126,7 +126,8 @@ static int read_pages(struct address_space *mapping, struct file *filp,
>>               struct page *page = list_to_page(pages);
>>               list_del(&page->lru);
>>               if (!add_to_page_cache_lru(page, mapping,
>> -                                     page->index, GFP_KERNEL)) {
>> +                                     page->index,
>> +                                     GFP_KERNEL | __GFP_NORETRY)) {
>>                       mapping->a_ops->readpage(filp, page);
>>               }
>>               page_cache_release(page);
>> --
>> 1.7.3.1
>>
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>
> --
> Kind regards,
> Minchan Kim

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