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@xxxxxxxxxx>
> Signed-off-by: Ying Han <yinghan@xxxxxxxxxx>
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.
> ---
> 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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