On Thu, Nov 23, 2023 at 10:57:22AM -0500, Gabriel Krisman Bertazi wrote: > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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.
