Re: [PATCH] nfsd: Prevent truncation of an unlinked inode from blocking access to its directory

["bfields@fieldses.org" <bfields@fieldses.org>]

On Fri, May 14, 2021 at 03:46:57PM +0000, Trond Myklebust wrote:
> On Fri, 2021-05-14 at 11:58 +0800, Nick Huang wrote:
> > From: Yu Hsiang Huang <nickhuang@synology.com>
> > 
> > Truncation of an unlinked inode may take a long time for I/O waiting,
> > and
> > it doesn't have to prevent access to the directory. Thus, let
> > truncation
> > occur outside the directory's mutex, just like do_unlinkat() does.
> > 
> > Signed-off-by: Yu Hsiang Huang <nickhuang@synology.com>
> > Signed-off-by: Bing Jing Chang <bingjingc@synology.com>
> > Signed-off-by: Robbie Ko <robbieko@synology.com>
> > ---
> >  fs/nfsd/vfs.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > index 15adf1f6ab21..39948f130712 100644
> > --- a/fs/nfsd/vfs.c
> > +++ b/fs/nfsd/vfs.c
> > @@ -1859,6 +1859,7 @@ nfsd_unlink(struct svc_rqst *rqstp, struct
> > svc_fh *fhp, int type,
> >  {
> >         struct dentry   *dentry, *rdentry;
> >         struct inode    *dirp;
> > +       struct inode    *rinode;
> >         __be32          err;
> >         int             host_err;
> >  
> > @@ -1887,6 +1888,8 @@ nfsd_unlink(struct svc_rqst *rqstp, struct
> > svc_fh *fhp, int type,
> >                 host_err = -ENOENT;
> >                 goto out_drop_write;
> >         }
> > +       rinode = d_inode(rdentry);
> > +       ihold(rinode);
> >  
> >         if (!type)
> >                 type = d_inode(rdentry)->i_mode & S_IFMT;
> > @@ -1902,6 +1905,8 @@ nfsd_unlink(struct svc_rqst *rqstp, struct
> > svc_fh *fhp, int type,
> >         if (!host_err)
> >                 host_err = commit_metadata(fhp);
> 
> Why leave the commit_metadata() call under the lock? If you're
> concerned about latency, then it makes more sense to call fh_unlock()
> before flushing those metadata updates to disk.
> 
> This is, BTW, an optimisation that appears to be possible in several
> other cases in fs/nfsd/vfs.c.

I'm tentatively applying the original patch plus the following.

Create and rename code are two other places where we have
commit_metadata() calls that could probably be moved out from under
locks but they looked slightly more complicated.

--b.

commit ec79990df716
Author: J. Bruce Fields <bfields@redhat.com>
Date:   Fri May 14 18:21:37 2021 -0400

    nfsd: move some commit_metadata()s outside the inode lock
    
    The commit may be time-consuming and there's no need to hold the lock
    for it.
    
    More of these are possible, these were just some easy ones.
    
    Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 39948f130712..d73d3c9126fc 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1613,9 +1613,9 @@ nfsd_symlink(struct svc_rqst *rqstp, struct svc_fh *fhp,
 
 	host_err = vfs_symlink(&init_user_ns, d_inode(dentry), dnew, path);
 	err = nfserrno(host_err);
+	fh_unlock(fhp);
 	if (!err)
 		err = nfserrno(commit_metadata(fhp));
-	fh_unlock(fhp);
 
 	fh_drop_write(fhp);
 
@@ -1680,6 +1680,7 @@ nfsd_link(struct svc_rqst *rqstp, struct svc_fh *ffhp,
 	if (d_really_is_negative(dold))
 		goto out_dput;
 	host_err = vfs_link(dold, &init_user_ns, dirp, dnew, NULL);
+	fh_unlock(ffhp);
 	if (!host_err) {
 		err = nfserrno(commit_metadata(ffhp));
 		if (!err)
@@ -1902,10 +1903,10 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 		host_err = vfs_rmdir(&init_user_ns, dirp, rdentry);
 	}
 
+	fh_unlock(fhp);
 	if (!host_err)
 		host_err = commit_metadata(fhp);
 	dput(rdentry);
-	fh_unlock(fhp);
 	iput(rinode);    /* truncate the inode here */
 
 out_drop_write: