Re: [PATCH] ceph: is_root_ceph_dentry() cleanup

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Feb 11, 2025 at 12:08:21AM +0000, Viacheslav Dubeyko wrote:

> In general, the MDS can issue NULL dentries to clients so that  they "know" the
> direntry does not exist without having some capability (or lease) associated
> with it. As far as I can see, if application repeatedly does stat of file, then
> the kernel driver isn't repeatedly going out to the MDS to lookup that file. So,
> I assume that this is the goal of this check and logic.

Er...  On repeated stat(2) you will get ->lookup() called the first time around;
afterwards you'll be getting dentry from dcache lookup.  With ->d_revalidate()
each time you find it in dcache, and eviction if ->d_revalidate() says it's stale.
In which case a new dentry will be allocated and fed to ->lookup(), passed to
it in negative-unhashed state...