Re: [PATCH] xfs: fix I_DONTCACHE

[Dave Chinner <david@fromorbit.com>]

On Mon, Aug 23, 2021 at 07:58:41PM -0700, Darrick J. Wong wrote:
> On Tue, Aug 24, 2021 at 12:32:08PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > Yup, the VFS hoist broke it, and nobody noticed. Bulkstat workloads
> > make it clear that it doesn't work as it should.
> 
> Is there an easy way to test the dontcache behavior so that we don't
> screw this up again?
> 
> /me's brain is fried, will study this in more detail in the morning.

Perhaps. We can measure how many xfs inodes are cached via the
filesystem stats e.g.

$ pminfo -t xfs.vnodes.active
xfs.vnodes.active [number of vnodes not on free lists]
$ sudo grep xfs_inode /proc/slabinfo | awk '{ print $2 }'
243440
$ pminfo -f xfs.vnodes.active

xfs.vnodes.active
    value 243166
$

And so we should be able to run a bulkstat from fstests on a
filesystem with a known amount of files in it and measure the number
of cached inodes before/after...

I noticed this because I recently re-added the threaded per-ag
bulkstat scan to my scalability workload (via the xfs_io bulkstat
command) after I dropped it ages ago because per-ag threading of
fstests::src/bulkstat.c was really messy. It appears nobody has
been paying attention to bulkstat memory usage (and therefore
I_DONTCACHE behaviour) for some time....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com