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Subject: Re: [PATCH RFC] NFSD: synchronously unhash a file on NFSv4 CLOSE
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <20200914205127.GD30007@fieldses.org>
Date:   Mon, 14 Sep 2020 16:53:15 -0400
Cc:     Trond Myklebust <trond.myklebust@primarydata.com>,
        Jeff Layton <jlayton@redhat.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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References: <159976711218.1334.14422329830210182280.stgit@klimt.1015granger.net>
 <20200914205127.GD30007@fieldses.org>
To:     Bruce Fields <bfields@fieldses.org>
Sender: linux-nfs-owner@vger.kernel.org
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> On Sep 14, 2020, at 4:51 PM, J. Bruce Fields <bfields@fieldses.org> =
wrote:
>=20
> On Thu, Sep 10, 2020 at 03:45:12PM -0400, Chuck Lever wrote:
>> I recently observed a significant slowdown in a long-running
>> test on NFSv4.0 mounts.
>>=20
>> An OPEN for write seems to block the NFS server from offering
>> delegations on that file for a few seconds. The problem is that
>> when that file is closed, the filecache retains an open-for-write
>> file descriptor until the laundrette runs. That keeps the inode's
>> i_writecount positive until the cached file is finally unhashed
>> and closed.
>>=20
>> Force the NFSv4 CLOSE logic to call filp_close() to eliminate
>> the underlying cached open-for-write file as soon as the client
>> closes the file.
>>=20
>> This minor change claws back about 80% of the performance
>> regression.
>=20
> That's really useful to know.  But mainly this makes me think that
> nfsd4_check_conflicting_opens() is wrong.
>=20
> I'm trying to determine whether a given file has a non-nfsd writer by
> counting the number of write opens nfsd holds on a given file and
> subtracting that from i_writecount.
>=20
> But the way I'm counting write opens probably isn't correct.

I entertained that possibility, but I couldn't figure out
a better way to count opens-for-write.


> --b.
>=20
>>=20
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>> fs/nfsd/nfs4state.c |    8 ++++++--
>> 1 file changed, 6 insertions(+), 2 deletions(-)
>>=20
>> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
>> index 3ac40ba7efe1..0b3059b8b36c 100644
>> --- a/fs/nfsd/nfs4state.c
>> +++ b/fs/nfsd/nfs4state.c
>> @@ -613,10 +613,14 @@ static void __nfs4_file_put_access(struct =
nfs4_file *fp, int oflag)
>> 		if (atomic_read(&fp->fi_access[1 - oflag]) =3D=3D 0)
>> 			swap(f2, fp->fi_fds[O_RDWR]);
>> 		spin_unlock(&fp->fi_lock);
>> -		if (f1)
>> +		if (f1) {
>> +			=
nfsd_file_close_inode_sync(locks_inode(f1->nf_file));
>> 			nfsd_file_put(f1);
>> -		if (f2)
>> +		}
>> +		if (f2) {
>> +			=
nfsd_file_close_inode_sync(locks_inode(f2->nf_file));
>> 			nfsd_file_put(f2);
>> +		}
>> 	}
>> }

--
Chuck Lever