linux-kernel - Re: OOM: Better, but still there on 4.9

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Date:   Sat, 17 Dec 2016 00:31:11 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Chris Mason <clm@...com>
Cc:     Nils Holland <nholland@...ys.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, David Sterba <dsterba@...e.cz>,
        linux-btrfs@...r.kernel.org
Subject: Re: OOM: Better, but still there on 4.9

On Fri 16-12-16 17:47:25, Chris Mason wrote:
> On 12/16/2016 05:14 PM, Michal Hocko wrote:
> > On Fri 16-12-16 13:15:18, Chris Mason wrote:
> > > On 12/16/2016 02:39 AM, Michal Hocko wrote:
> > [...]
> > > > I believe the right way to go around this is to pursue what I've started
> > > > in [1]. I will try to prepare something for testing today for you. Stay
> > > > tuned. But I would be really happy if somebody from the btrfs camp could
> > > > check the NOFS aspect of this allocation. We have already seen
> > > > allocation stalls from this path quite recently
> > > 
> > > Just double checking, are you asking why we're using GFP_NOFS to avoid going
> > > into btrfs from the btrfs writepages call, or are you asking why we aren't
> > > allowing highmem?
> > 
> > I am more interested in the NOFS part. Why cannot this be a full
> > GFP_KERNEL context? What kind of locks we would lock up when recursing
> > to the fs via slab shrinkers?
> > 
> 
> Since this is our writepages call, any jump into direct reclaim would go to
> writepage, which would end up calling the same set of code to read metadata
> blocks, which would do a GFP_KERNEL allocation and end up back in writepage
> again.

But we are not doing pageout on the page cache from the direct reclaim
for a long time. So basically the only way to recurse back to the fs
code is via slab ([di]cache) shrinkers. Are those a problem as well?

-- 
Michal Hocko
SUSE Labs