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

Trond Myklebust
2021-05-18T18:36:28Z

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.

>
> 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