Re: [PATCH 0/2] dcache: don't leave negative dentries in casefolded directories

[Gabriel Krisman Bertazi <krisman@suse.de>]

Christian Brauner <brauner@kernel.org> writes:

> ext4, f2fs and tmpfs keep negative dentries out of casefolded
> directories. A negative dentry remembers one spelling of the name and
> the next create() through it would write that spelling to disk.
>
> So their ->lookup() doesn't cache a miss and their ->unlink() and
> ->rmdir() call d_invalidate() on the dentry so that d_delete() doesn't
> turn it into a negative dentry.
>
> But d_invalidate() is the wrong primitive for that. It's for a name that
> went stale. It walks the subtree, prunes it and detaches every mount it
> finds, under namespace_sem, and here that runs from inside the
> filesystem's own directory operation. ext4_rmdir() does it with the
> journal handle still open and ext4_unlink() does it even when the
> unlink failed. All anyone wanted is the unhash.
>
> So d_delete() now unhashes the dentry when the parent is casefolded,
> next to the dentry-negative sysctl that does the same for every
> directory. ext4, f2fs and tmpfs drop their d_invalidate() calls.
>
> xfs's ascii-ci doesn't set S_CASEFOLD and can't so it d_drop()s instead.
> I did consider setting S_CASEFOLD on ascii-ci directories and didn't.
> overlayfs warns when a layer's casefold flag disagrees with the
> superblock having an encoding, and xfs has none.
>
> Nothing changes for userspace. A miss still isn't cached and a removed
> name still leaves no negative dentry behind. Verified with the same
> create, unlink, create, readdir and stat sequence in a plain and in a
> casefolded directory on tmpfs and on ext4, before and after.

I think the change looks fine and it passed fstests.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>

Of course, the better approach would be to enable negative dentries
but the last attempt went sideways, though not for any good reason.

thanks!

-- 
Gabriel Krisman Bertazi