netdev - Re: ppoll() stuck on POLLIN while TCP peer is sending

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Message-ID: <20130109134248.GE13304@suse.de>
Date:	Wed, 9 Jan 2013 13:42:48 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Eric Wong <normalperson@...t.net>, linux-mm@...ck.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: ppoll() stuck on POLLIN while TCP peer is sending

On Tue, Jan 08, 2013 at 06:32:29PM -0800, Eric Dumazet wrote:
> On Tue, 2013-01-08 at 18:14 -0800, Eric Dumazet wrote:
> > On Tue, 2013-01-08 at 23:23 +0000, Eric Wong wrote:
> > > Mel Gorman <mgorman@...e.de> wrote:
> > > > Please try the following patch. However, even if it works the benefit of
> > > > capture may be so marginal that partially reverting it and simplifying
> > > > compaction.c is the better decision.
> > > 
> > > I already got my VM stuck on this one.  I had two twosleepy instances,
> > > 2774 was the one that got stuck (also confirmed by watching top).
> > > 
> > > Btw, have you been able to reproduce this on your end?
> > > 
> > > I think the easiest reproduction on my 2-core VM is by running 2
> > > twosleepy processes and doing the following to dirty a lot of pages:
> > 
> > Given the persistent sk_stream_wait_memory() traces I suspect a plain
> > TCP bug, triggered by some extra wait somewhere.
> > 
> > Please mm guys don't spend too much time right now, I'll try to
> > reproduce the problem.
> > 
> > Don't be confused by sk_stream_wait_memory() name.
> > A thread is stuck here because TCP stack is failing to wake it.
> > 
> 
> Hmm, it seems sk_filter() can return -ENOMEM because skb has the
> pfmemalloc() set.
> 

The skb should not have pfmemalloc set in most cases, particularly after
cfd19c5a (mm: only set page->pfmemalloc when ALLOC_NO_WATERMARKS was used)
but the capture patch also failed to clear pfmemalloc properly so it could
be set in error.

-- 
Mel Gorman
SUSE Labs
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