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

bfields
2021-05-18T18:21:18Z

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?

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.