Re: [PATCH] improve the performance of large sequential write NFSworkloads

[Jan Kara]

On Wed 23-12-09 03:43:02, Christoph Hellwig wrote:
> On Tue, Dec 22, 2009 at 01:35:39PM +0100, Jan Kara wrote:
> > >    nfsd_sync:
> > >      [take i_mutex]
> > >        filemap_fdatawrite  => can also be blocked, but less a problem
> > >      [drop i_mutex]
> > >        filemap_fdatawait
> > >  
> > >    Maybe it's a dumb question, but what's the purpose of i_mutex here?
> > >    For correctness or to prevent livelock? I can imagine some livelock
> > >    problem here (current implementation can easily wait for extra
> > >    pages), however not too hard to fix.
> >   Generally, most filesystems take i_mutex during fsync to
> > a) avoid all sorts of livelocking problems
> > b) serialize fsyncs for one inode (mostly for simplicity)
> >   I don't see what advantage would it bring that we get rid of i_mutex
> > for fdatawait - only that maybe writers could proceed while we are
> > waiting but is that really the problem?
> 
> It would match what we do in vfs_fsync for the non-nfsd path, so it's
> a no-brainer to do it.  In fact I did switch it over to vfs_fsync a
> while ago but that go reverted because it caused deadlocks for
> nfsd_sync_dir which for some reason can't take the i_mutex (I'd have to
> check the archives why).
> 
> Here's a RFC patch to make some more sense of the fsync callers in nfsd,
> including fixing up the data write/wait calling conventions to match the
> regular fsync path (which might make this a -stable candidate):
  The patch looks good to me from general soundness point of view :).
Someone with more NFS knowledge should tell whether dropping i_mutex for
fdatawrite_and_wait is fine for NFS.

								Honza
 
> Index: linux-2.6/fs/nfsd/vfs.c
> ===================================================================
> --- linux-2.6.orig/fs/nfsd/vfs.c	2009-12-23 09:32:45.693170043 +0100
> +++ linux-2.6/fs/nfsd/vfs.c	2009-12-23 09:39:47.627170082 +0100
> @@ -769,45 +769,27 @@ nfsd_close(struct file *filp)
>  }
>  
>  /*
> - * Sync a file
> - * As this calls fsync (not fdatasync) there is no need for a write_inode
> - * after it.
> + * Sync a directory to disk.
> + *
> + * This is odd compared to all other fsync callers because we
> + *
> + *  a) do not have a file struct available
> + *  b) expect to have i_mutex already held by the caller
>   */
> -static inline int nfsd_dosync(struct file *filp, struct dentry *dp,
> -			      const struct file_operations *fop)
> +int
> +nfsd_sync_dir(struct dentry *dentry)
>  {
> -	struct inode *inode = dp->d_inode;
> -	int (*fsync) (struct file *, struct dentry *, int);
> +	struct inode *inode = dentry->d_inode;
>  	int err;
>  
> -	err = filemap_fdatawrite(inode->i_mapping);
> -	if (err == 0 && fop && (fsync = fop->fsync))
> -		err = fsync(filp, dp, 0);
> -	if (err == 0)
> -		err = filemap_fdatawait(inode->i_mapping);
> +	WARN_ON(!mutex_is_locked(&inode->i_mutex));
>  
> +	err = filemap_write_and_wait(inode->i_mapping);
> +	if (err == 0 && inode->i_fop->fsync)
> +		err = inode->i_fop->fsync(NULL, dentry, 0);
>  	return err;
>  }
>  
> -static int
> -nfsd_sync(struct file *filp)
> -{
> -        int err;
> -	struct inode *inode = filp->f_path.dentry->d_inode;
> -	dprintk("nfsd: sync file %s\n", filp->f_path.dentry->d_name.name);
> -	mutex_lock(&inode->i_mutex);
> -	err=nfsd_dosync(filp, filp->f_path.dentry, filp->f_op);
> -	mutex_unlock(&inode->i_mutex);
> -
> -	return err;
> -}
> -
> -int
> -nfsd_sync_dir(struct dentry *dp)
> -{
> -	return nfsd_dosync(NULL, dp, dp->d_inode->i_fop);
> -}
> -
>  /*
>   * Obtain the readahead parameters for the file
>   * specified by (dev, ino).
> @@ -1011,7 +993,7 @@ static int wait_for_concurrent_writes(st
>  
>  	if (inode->i_state & I_DIRTY) {
>  		dprintk("nfsd: write sync %d\n", task_pid_nr(current));
> -		err = nfsd_sync(file);
> +		err = vfs_fsync(file, file->f_path.dentry, 0);
>  	}
>  	last_ino = inode->i_ino;
>  	last_dev = inode->i_sb->s_dev;
> @@ -1180,7 +1162,7 @@ nfsd_commit(struct svc_rqst *rqstp, stru
>  		return err;
>  	if (EX_ISSYNC(fhp->fh_export)) {
>  		if (file->f_op && file->f_op->fsync) {
> -			err = nfserrno(nfsd_sync(file));
> +			err = nfserrno(vfs_fsync(file, file->f_path.dentry, 0));
>  		} else {
>  			err = nfserr_notsupp;
>  		}
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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