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 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.