Re: [Linux-cachefs] [BUG] fscache writing but not reading

[David Wysochanski <dwysocha@redhat.com>]

On Fri, May 19, 2023 at 7:53 AM David Wysochanski <dwysocha@redhat.com> wrote:
>
> On Thu, May 18, 2023 at 4:21 PM Chris Chilvers <chilversc@gmail.com> wrote:
> >
> > On Tue, 16 May 2023 at 20:28, David Wysochanski <dwysocha@redhat.com> wrote:
> > >
> > > On Tue, May 16, 2023 at 11:42 AM Chris Chilvers <chilversc@gmail.com> wrote:
> > > >
> > > > While testing the fscache performance fixes [1] that were merged into 6.4-rc1
> > > > it appears that the caching no longer works. The client will write to the cache
> > > > but never reads.
> > > >
> > > Thanks for the report.
> > >
> > > If you reboot do you see reads from the cache?
> >
> > On the first read after a reboot it uses the cache, but subsequent
> > reads do not use the cache.
> >
> > > You can check if the cache is being read from by looking in
> > > /proc/fs/fscache/stats
> > > at the "IO" line:
> > > # grep IO /proc/fs/fscache/stats
> > > IO     : rd=80030 wr=0
> >
> > Running the tests 4 times (twice before reboot, and twice after) give
> > the following metrics:
> > FS-Cache I/O (delta)
> > Run       rd        wr
> >  1         0    39,250
> >  2       130    38,894
> >  3    39,000         0
> >  4        72    38,991
> >
> > > Can you share:
> > > 1. NFS server you're using (is it localhost or something else)
> > > 2. NFS version
> >
> > The NFS server and client are separate VMs on the same network.
> > Server NFS version: Ubuntu 22.04 jammy, kernel 5.15.0-1021-gcp
> > Client NFS version: Ubuntu 22.04 jammy, kernel 6.4.0-060400rc1-generic
> > (https://kernel.ubuntu.com/~kernel-ppa/mainline/v6.4-rc1/)
> > Client nfs-utils: 2.6.3-rc6
> > Client cachefilesd: 0.10.10-0.2ubuntu1
> >
> > > In addition to checking the above for the reads from the cache, you can also
> > > see whether NFS reads are going over the wire pretty easily with a similar
> > > technique.
> > >
> > > Copy /proc/self/mounstats to a file before your test, then make a second copy
> > > after the test, then run mountstats as follows:
> > > mountstats -S /tmp/mountstats.1 -f /tmp/mountstats.2
> >
> > app read    = applications read bytes via read(2)
> > client read = client read bytes via NFS READ
> >
> > Run           app read        client read
> >  1     322,122,547,200    322,122,547,200
> >  2     322,122,547,200    321,048,805,376
> >  3     322,122,547,200                  0
> >  4     322,122,547,200    321,593,053,184
> >
> > I've put the full data in a GitHub gist, along with a graph collected
> > from a metrics agent:
> > https://gist.github.com/chilversc/54eb76155ad37b66cb85186e7449beaa
> > https://gist.githubusercontent.com/chilversc/54eb76155ad37b66cb85186e7449beaa/raw/09828c596d0cfc44bc0eb67f40e4033db202326e/metrics.png
> >
>
> Thanks Chris for all this info.  I see you're using NFSv3 so I'll
> focus on that, and review all this info for clues.
> I also have been working on some updated test cases and see some very
> unusual behavior like you're reporting.
>
> I also confirmed that adding the two patches for "Issue #1"  onto
> 6.4-rc1 resolve _most_ of the caching issues.
> However, even after those patches, in some limited instances, there
> are still NFS reads over the wire when there should only be reads from
> the cache.
> There may be multiple bugs here.

I actually misspoke regarding "multiple bugs", as I forgot to add a
small NFS hunk (see below) needed to dhowells 2nd patch (v6 of mm,
netfs, fscache: Stop read optimisation when folio removed from
pagecache).
After the below small hunk was added on top of dhowells 2nd patch, all
my tests pass.
I've also enhanced my existing tests to check NFS READs, fscache
READs, and fscache WRITEs as expected.
And I added an additional test to create files the size of RAM, read
them multiple times, and check for the ops are as expected.
So I'm confident if we add dhowells 2 patches, plus the below hunk for
NFS, these caching issues will be resolved.

diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c
index 8c35d88a84b1..d4a20748b14f 100644
--- a/fs/nfs/fscache.c
+++ b/fs/nfs/fscache.c
@@ -180,6 +180,10 @@ void nfs_fscache_init_inode(struct inode *inode)
                                               &auxdata,      /* aux_data */
                                               sizeof(auxdata),
                                               i_size_read(inode));
+
+       if (netfs_inode(inode)->cache)
+               mapping_set_release_always(inode->i_mapping);
+
 }

 /*