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

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Date: Thu, 23 Nov 2023 05:09:43 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Christian Brauner <brauner@...nel.org>,
	Gabriel Krisman Bertazi <krisman@...e.de>, tytso@....edu,
	linux-f2fs-devel@...ts.sourceforge.net, ebiggers@...nel.org,
	linux-fsdevel@...r.kernel.org, jaegeuk@...nel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on
 case-insensitive ext4 and f2fs

On Wed, Nov 22, 2023 at 04:18:56PM -0800, Linus Torvalds wrote:
> On Wed, 22 Nov 2023 at 13:19, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > The serious gap, AFAICS, is the interplay with open-by-fhandle.
> 
> So I'm obviously not a fan of igncase filesystems, but I don't think
> this series actually changes any of that.
> 
> > It's not unfixable, but we need to figure out what to do when
> > lookup runs into a disconnected directory alias.  d_splice_alias()
> > will move it in place, all right, but any state ->lookup() has
> > hung off the dentry that had been passed to it will be lost.
> 
> I guess this migth be about the new DCACHE_CASEFOLDED_NAME bit.
> 
> At least for now, that is only used by generic_ci_d_revalidate() for
> negative dentries, so it shouldn't matter for that d_splice_alias()
> that only does positive dentries. No?
> 
> Or is there something else you worry about?

Dentries created by d_obtain_alias() will never go anywhere near
generic_set_encrypted_ci_d_ops().  They do *not* get ->d_op set
that way.  When ext4_lookup() does a lookup in c-i directory it
does have ->d_op set on dentry it got from the caller.  Which is
promptly discarded when d_splice_alias() finds a preexisting
alias for it.

Positive dentries eventually become negative; not invalidating them
when that happens is a large part of the point of this series.
->d_revalidate() is taught to check if they are marked with that
bit, but... they won't have that ->d_revalidate() associated with
them, will they?  ->d_hash() and ->d_compare() come from the
parent, but ->d_revalidate() comes from dentry itself.

In other words, generic_ci_d_revalidate() won't see the lack of
that bit on dentry, etc. - it won't be called for that dentry
in the first place.