Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on case-insensitive ext4 and f2fs

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

On Thu, Nov 23, 2023 at 10:57:22AM -0500, Gabriel Krisman Bertazi wrote:
> Linus Torvalds <torvalds@linux-foundation.org> writes:
> 
> > Side note: Gabriel, as things are now, instead of that
> >
> >         if (!d_is_casefolded_name(dentry))
> >                 return 0;
> >
> > in generic_ci_d_revalidate(), I would suggest that any time a
> > directory is turned into a case-folded one, you'd just walk all the
> > dentries for that directory and invalidate negative ones at that
> > point. Or was there some reason I missed that made it a good idea to
> > do it at run-time after-the-fact?
> >
> 
> The problem I found with that approach, which I originally tried, was
> preventing concurrent lookups from racing with the invalidation and
> creating more 'case-sensitive' negative dentries.  Did I miss a way to
> synchronize with concurrent lookups of the children of the dentry?  We
> can trivially ensure the dentry doesn't have positive children by
> holding the parent lock, but that doesn't protect from concurrent
> lookups creating negative dentries, as far as I understand.

AFAICS, there is a problem with dentries that never came through
->lookup().  Unless I'm completely misreading your code, your
generic_ci_d_revalidate() is not called for them.  Ever.

Hash lookups are controlled by ->d_op of parent; that's where ->d_hash()
and ->d_compare() come from.  Revalidate comes from *child*.  You need
->d_op->d_revalidate of child dentry to be set to your generic_ci_d_revalidate().

The place where it gets set is generic_set_encrypted_ci_d_ops().  Look
at its callchain; in case of ext4 it gets called from ext4_lookup_dentry(),
which is called from ext4_lookup().  And dentry passed to it is the
argument of ->lookup().

Now take a look at open-by-fhandle stuff; all methods in there
(->fh_to_dentry(), ->fh_to_parent(), ->get_parent()) end up
returning d_obtain_alias(some inode).

We *do* call ->lookup(), all right - in reconnect_one(), while
trying to connect those suckers with the main tree.  But the way
it works is that d_splice_alias() in ext4_lookup() moves the
existing alias for subdirectory, connecting it to the parent.
That's not the dentry ext4_lookup() had set ->d_op on - that's
the dentry that came from d_obtain_alias().  And those do not
have ->d_op set by anything in your tree.

That's the problem I'd been talking about - there is a class of situations
where the work done by ext4_lookup() to set the state of dentry gets
completely lost.  After lookup you do have a dentry in the right place,
with the right name and inode, etc., but with NULL ->d_op->d_revalidate.