Re: linux-next: slab shrinkers: BUG at mm/list_lru.c:92

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 11 Jul 2013 12:26:34 +1000 Dave Chinner <david@fromorbit.com> wrote:

> > Just for reference. wait_on_page_writeback is issued only for memcg
> > reclaim because there is no other throttling mechanism to prevent from
> > too many dirty pages on the list, thus pre-mature OOM killer. See
> > e62e384e9d (memcg: prevent OOM with too many dirty pages) for more
> > details. The original patch relied on may_enter_fs but that check
> > disappeared by later changes by c3b94f44fc (memcg: further prevent OOM
> > with too many dirty pages).
> 
> Aye. That's the exact code I was looking at yesterday and wondering
> "how the hell is waiting on page writeback valid in GFP_NOFS
> context?". It seems that memcg reclaim is intentionally ignoring
> GFP_NOFS to avoid OOM issues.  That's a memcg implementation problem,
> not a filesystem or LRU infrastructure problem....

Yup, c3b94f44fc shouldn't have done that.

Throttling by waiting on a specific page is indeed prone to deadlocks
and has a number of efficiency problems as well: if 1,000,000 pages
came clean while you're waiting for *this* page to come clean, you're
left looking pretty stupid.

Hence congestion_wait(), which perhaps can save us here.  I'm not sure
how the wait_on_page_writeback() got back in there - I must have been
asleep at the time.