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

[Dave Chinner]

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....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx