linux-kernel - Re: [RFC PATCH] mm, compaction: allow compaction for GFP

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Date:   Wed, 5 Oct 2016 13:38:45 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Joonsoo Kim <js1304@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm, compaction: allow compaction for GFP_NOFS
 requests

On Wed 05-10-16 07:32:02, Dave Chinner wrote:
> On Tue, Oct 04, 2016 at 10:12:15AM +0200, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > compaction has been disabled for GFP_NOFS and GFP_NOIO requests since
> > the direct compaction was introduced by 56de7263fcf3 ("mm: compaction:
> > direct compact when a high-order allocation fails"). The main reason
> > is that the migration of page cache pages might recurse back to fs/io
> > layer and we could potentially deadlock. This is overly conservative
> > because all the anonymous memory is migrateable in the GFP_NOFS context
> > just fine.  This might be a large portion of the memory in many/most
> > workkloads.
> > 
> > Remove the GFP_NOFS restriction and make sure that we skip all fs pages
> > (those with a mapping) while isolating pages to be migrated. We cannot
> > consider clean fs pages because they might need a metadata update so
> > only isolate pages without any mapping for nofs requests.
> > 
> > The effect of this patch will be probably very limited in many/most
> > workloads because higher order GFP_NOFS requests are quite rare,
> 
> You say they are rare only because you don't know how to trigger
> them easily.  :/

true

> Try this:
> 
> # mkfs.xfs -f -n size=64k <dev>
> # mount <dev> /mnt/scratch
> # time ./fs_mark  -D  10000  -S0  -n  100000  -s  0  -L  32 \
>         -d  /mnt/scratch/0  -d  /mnt/scratch/1 \
>         -d  /mnt/scratch/2  -d  /mnt/scratch/3 \
>         -d  /mnt/scratch/4  -d  /mnt/scratch/5 \
>         -d  /mnt/scratch/6  -d  /mnt/scratch/7 \
>         -d  /mnt/scratch/8  -d  /mnt/scratch/9 \
>         -d  /mnt/scratch/10  -d  /mnt/scratch/11 \
>         -d  /mnt/scratch/12  -d  /mnt/scratch/13 \
>         -d  /mnt/scratch/14  -d  /mnt/scratch/15

Does this simulate a standard or usual fs workload/configuration?  I am
not questioning that higher order NOFS allocations are non-existent -
that's why I came with the patch in the first place ;). My observation
was that they are so rare that the visible effect of this patch might be
quite low or even hard to notice.

Anyway, thanks for a _useful_ testcase to play with! Let's see what
numbers I get from this.

-- 
Michal Hocko
SUSE Labs