linux-kernel - Re: [regression 4.2-rc3] loop: xfstests xfs/073 deadlocked in low memory conditions

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Message-ID: <20150729221356.GC16638@dastard>
Date:	Thu, 30 Jul 2015 08:13:56 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Ming Lei <ming.lei@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <andreas.dilger@...el.com>,
	Oleg Drokin <oleg.drokin@...el.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, xfs@....sgi.com, linux-nfs@...r.kernel.org,
	linux-cifs@...r.kernel.org
Subject: Re: [regression 4.2-rc3] loop: xfstests xfs/073 deadlocked in low
 memory conditions

On Wed, Jul 29, 2015 at 01:54:12PM +0200, Michal Hocko wrote:
> On Tue 21-07-15 10:58:59, Michal Hocko wrote:
> > [CCing more people from a potentially affected fs - the reference to the 
> >  email thread is: http://marc.info/?l=linux-mm&m=143744398020147&w=2]
...
> > > The didn't used to happen, because the loop device used to issue
> > > reads through the splice path and that does:
> > > 
> > > 	error = add_to_page_cache_lru(page, mapping, index,
> > > 			GFP_KERNEL & mapping_gfp_mask(mapping));
> > > 
> > > i.e. it pays attention to the allocation context placed on the
> > > inode and so is doing GFP_NOFS allocations here and avoiding the
> > > recursion problem.
> > > 
> > > [ CC'd Michal Hocko and the mm list because it's a clear exaple of
> > > why ignoring the mapping gfp mask on any page cache allocation is
> > > a landmine waiting to be tripped over. ]
> > 
> > Thank you for CCing me. I haven't noticed this one when checking for
> > other similar hardcoded GFP_KERNEL users (6afdb859b710 ("mm: do not
> > ignore mapping_gfp_mask in page cache allocation paths")). And there
> > seem to be more of them now that I am looking closer.
> > 
> > I am not sure what to do about fs/nfs/dir.c:nfs_symlink which doesn't
> > require GFP_NOFS or mapping gfp mask for other allocations in the same
> > context.
> > 
> > What do you think about this preliminary (and untested) patch?
> 
> Dave, did you have chance to test the patch in your environment? Is the
> patch good to go or we want a larger refactoring?

No, I haven't had a chance to test it yet. I'll try to get somethign
done by the end of the week, but I'm not able to reliably
reproduce the hang I saw (i.e. the analysis I did was from the first
deadlock and I've only seen it once since) so testing is likely to
be inconclusive, anyway....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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