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From:   "Benjamin Coddington" <bcodding@redhat.com>
To:     "Trond Myklebust" <trondmy@hammerspace.com>
Cc:     linux-nfs@vger.kernel.org
Subject: Lookup revalidation for OPEN_CLAIM_FH
Date:   Thu, 16 Jan 2020 08:51:22 -0500
Message-ID: <31B20BC3-A089-47F9-9821-7A3543FF7413@redhat.com>
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Sender: linux-nfs-owner@vger.kernel.org
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Hi Trond,

I'd like to fix up lookup revalidation for v4.1+ when the client is using
OPEN_CLAIM_FH.  The fixes a while back for Stan Hu's case do not seem to
improve things for v4.1, and actually make the behavior a bit worse since we
no longer pass through nfs_lookup_verify_inode(), which would catch the
cases where nlink == 0.

Would you accept work to _always_ revalidate the dentry's parent for
CLAIM_FH?  Alternatively, it seems that CLAIM_NULL would be preferable for
this case, though I don't know how the client would know when to decide
between them.

Here's a simple reproducer for convenience, I think we've already all agreed
that the behavior we want is for the second open by `cat` to reflect the
results of the move on the server, or at least eventually later opens would
revalidate the dentry:

#!/bin/bash

set -o xtrace
vers=4.1

exportfs -ua
exportfs -o rw,sec=sys,no_root_squash *:/exports

mkdir /mnt/localhost || true

rm -f /exports/file{1,2}

echo this is file 1 > /exports/file1
echo this is file 2 > /exports/file2

mount -t nfs -ov$vers,sec=sys localhost:/exports /mnt/localhost

tail -f /mnt/localhost/file1 &
sleep 1

# this is file 1
cat /mnt/localhost/file1

# overwrite the file on the server:
mv -f /exports/file2 /exports/file1

# this is file 2
cat /mnt/localhost/file1

killall tail
# this is file 2
cat /mnt/localhost/file1
umount /mnt/localhost


Switching the $vers variable between v4.0 and v4.1 in this script shows the
difference in behavior.

Thanks for any advice,
Ben