Re: [RFC PATCH] mm, compaction: allow compaction for GFP_NOFS requests

Michal Hocko
2016-10-05T11:38:49Z

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@suse.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