Re: [PATCH RFC] NFSD: synchronously unhash a file on NFSv4 CLOSE

["J. Bruce Fields" <bfields@fieldses.org>]

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.
> 
> 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.
> 
> 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.
> 
> This minor change claws back about 80% of the performance
> regression.

That's really useful to know.  But mainly this makes me think that
nfsd4_check_conflicting_opens() is wrong.

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.

But the way I'm counting write opens probably isn't correct.

--b.

> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs4state.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> 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]) == 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);
> +		}
>  	}
>  }
>  
>