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

From: Al Viro <viro@zeniv.linux.org.uk>
To: Gabriel Krisman Bertazi <gabriel@krisman.be>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Christian Brauner <brauner@kernel.org>,
	tytso@mit.edu, linux-f2fs-devel@lists.sourceforge.net,
	ebiggers@kernel.org, linux-fsdevel@vger.kernel.org,
	jaegeuk@kernel.org, linux-ext4@vger.kernel.org
Subject: Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on case-insensitive ext4 and f2fs
Date: Thu, 23 Nov 2023 19:53:27 +0000	[thread overview]
Message-ID: <20231123195327.GP38156@ZenIV> (raw)
In-Reply-To: <87bkbki91c.fsf@>

On Thu, Nov 23, 2023 at 02:06:39PM -0500, Gabriel Krisman Bertazi wrote:

> > A paragraph above you've said that it's not constant over the entire
> > filesystem.
> 
> The same ->d_op is used by every dentry in the filesystem if the superblock
> has the casefold bit enabled, regardless of whether a specific inode is
> casefolded or not. See generic_set_encrypted_ci_d_ops in my tree. It is
> called unconditionally by ext4_lookup and only checks the superblock:
> 
> void generic_set_encrypted_ci_d_ops(struct dentry *dentry)
> {
>         if (dentry->d_sb->s_encoding) {
> 		d_set_d_op(dentry, &generic_encrypted_ci_dentry_ops);
> 		return;
> 	}
>         ...
> 
> What I meant was that this used to be set once at sb->s_d_op, and
> propagated during dentry allocation.  Therefore, the propagation to the
> alias would happen inside __d_alloc.  Once we enabled fscrypt and
> casefold to work together, sb->s_d_op is NULL

Why?  That's what I don't understand - if you really want it for
all dentries on that filesystem, that's what ->s_d_op is for.
If it is not, you have that problem, no matter which way you flip ->d_op
value.

> and we always set the same
> handler for every dentry during lookup.

Not every dentry goes through lookup - see upthread for details.

> > Look, it's really simple - any setup work of that sort done in ->lookup()
> > is either misplaced, or should be somehow transferred over to the alias
> > if one gets picked.
> >
> > As for d_obtain_alias()... AFAICS, it's far more limited in what information
> > it could access.  It knows the inode, but it has no idea about the parent
> > to be.
> 
> Since it has the inode, d_obtain_alias has the superblock.  I think that's all
> we need for generic_set_encrypted_ci_d_ops.

Huh?  If it really depends only upon the superblock, just set it in ->s_d_op
when you set the superblock up.

Again, whatever setup you do for dentry in ->lookup(), you either
	* have a filesystem that never picks an existing directory alias
(e.g. doesn't allow open-by-fhandle or has a very unusual implementation
of related methods, like e.g. shmem), or
	* have that setup misplaced, in part that applies to all dentries out
there (->s_d_op for universal ->d_op value, ->d_init() for uniform allocation
of objects hanging from ->d_fsdata and other things like that), or
	* need to figure out how to transfer the result to alias (manually
after d_splice_alias(), if races do not matter or using a new method explicitly
for that), or
	* lose that state for aliases.