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

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

On Tue, May 18, 2021 at 06:36:23PM +0000, Trond Myklebust wrote:
> On Tue, 2021-05-18 at 14:21 -0400, bfields@fieldses.org wrote:
> > On Tue, May 18, 2021 at 05:53:47PM +0000, Trond Myklebust wrote:
> > > On Sat, 2021-05-15 at 08:02 +0100, Christoph Hellwig wrote:
> > > > On Fri, May 14, 2021 at 03:46:57PM +0000, Trond Myklebust wrote:
> > > > > 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.
> > > > 
> > > > Also I'm not sure why the extra inode reference is needed.  What
> > > > speaks
> > > > against just moving the dput out of the locked section?
> > > 
> > > Isn't the inode reference taken just in order to ensure that the
> > > call
> > > to iput_final() (and in particular the call to
> > > truncate_inode_pages_final()) is performed outside the lock?
> > > 
> > > The dput() is presumably usually not particularly expensive, since
> > > the
> > > dentry is just a completely ordinary negative dentry at this point.
> > 
> > Right, but why bother with the extra ihold/iput instead of just
> > postponing the dput?
> 
> As I said above, the dentry is negative at this point. It doesn't carry
> a reference to the inode any more, so that wouldn't defer the inode
> truncation.

Oh, of course, thanks!

--b.

> 
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 79b0ff9b151e..aeed93c9874c 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -4084,7 +4084,6 @@ long do_unlinkat(int dfd, struct filename
> > *name)
> >                 inode = dentry->d_inode;
> >                 if (d_is_negative(dentry))
> >                         goto slashes;
> > -               ihold(inode);
> >                 error = security_path_unlink(&path, dentry);
> >                 if (error)
> >                         goto exit2;
> > @@ -4092,11 +4091,10 @@ long do_unlinkat(int dfd, struct filename
> > *name)
> >                 error = vfs_unlink(mnt_userns, path.dentry->d_inode,
> > dentry,
> >                                    &delegated_inode);
> >  exit2:
> > -               dput(dentry);
> >         }
> >         inode_unlock(path.dentry->d_inode);
> > -       if (inode)
> > -               iput(inode);    /* truncate the inode here */
> > +       if (!IS_ERR(dentry))
> > +               dput(dentry);   /* truncate the inode here */
> >         inode = NULL;
> >         if (delegated_inode) {
> >                 error = break_deleg_wait(&delegated_inode);
> > 
> > ?
> > 
> > --b.
> 
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
> 
>