linux-kernel - Re: [PATCH v3 2/3] fs: fuse: add backing

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Message-ID: <CAOQ4uxgM+oJxp0Od=i=Twj9EN2v2+rFByEKabZybic=6gA0QgA@mail.gmail.com>
Date: Mon, 12 May 2025 11:23:15 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Chen Linxuan <chenlinxuan@...ontech.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v3 2/3] fs: fuse: add backing_files control file

On Mon, May 12, 2025 at 10:27 AM Miklos Szeredi <miklos@...redi.hu> wrote:
>
> On Sun, 11 May 2025 at 11:56, Chen Linxuan <chenlinxuan@...ontech.com> wrote:
>
> > I noticed that the current extended attribute names already use the
> > namespace.value format.
> > Perhaps we could reuse this naming scheme and extend it to support
> > features like nested namespaces.
>
> Right.  Here's a link to an old and long thread about this:
>
>    https://lore.kernel.org/all/YnEeuw6fd1A8usjj@miu.piliscsaba.redhat.com/#r
>

The way I see it is, generic vs. specialized have pros and cons
There is no clear winner.
Therefore, investing time on the getxattr() direction without consensus
with vfs maintainer is not wise IMO.

> >
> > For instance, in a situation like this:
> >
> > A fixed file 0 in an io_uring is a FUSE fd.
> > This FUSE fd belongs to FUSE connection 64.
> > This FUSE fd has a backing file.
> > This backing file is actually provided by mnt_id=36.
> >
> > Running getfattr -m '-' /proc/path/to/the/io_uring/fd could return
> > something like:
> >
> > io_uring.fixed_files.0.fuse.conn="64"
> > io_uring.fixed_files.0.fuse.backing_file.mnt_id="36"
> > io_uring.fixed_files.0.fuse.backing_file.path="/path/to/real/file"
>
> Yeah, except listxattr wouldn't be able to properly work in such
> cases: it lacks support for hierarchy.
>
> The proposed solution was something like making getxattr on the
> "directory" return a listing of child object names.
>
> I.e. getxattr("/proc/123/fd/12", "io_uring.fixed_files.") would return
> the list of instantiated fixed file slots, etc...

The problem I see with this scheme is that it is not generic enough.
If lsof is to support displaying fuse backing files, then it needs to
know specifically about those magic xattrs.

Because lsof only displays information about open files, I think
it would be better to come up with a standard tag in fdinfo for lsof
to recognize, for example:

hidden_file: /path/to/hidden/file
hidden_files_list: /path/to/connections/N/backing_files

and then conform to the same fdinfo standard from fuse_dev_fd,
io_uring_fd, unix_socket_fd.

Making an interface more hierarchic than hidden_files_list:
is useless because lsof traverses all fds anyway to filter by
name pattern and I am very sceptical of anyone trying to
push for an API get_open_fds_by_name_pattern()...

Thanks,
Amir.