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

From: Dave Chinner
Date: Tue Oct 04 2016 - 16:32:49 EST

On Tue, Oct 04, 2016 at 10:12:15AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 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. :/

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

As soon as tail pushing on the journal starts (a few seconds in,
most likely), you'll start to see lots of 65kB allocations being
requested in GFP_NOFS context by the xfs-cil-worker context doing
journal checkpoint formatting....


Dave Chinner