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

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Message-ID: <CAOQ4uxiBLc9G+CvU-m5XMPbkFJLeCt6R86r8WaGEE2N3k9_qrw@mail.gmail.com>
Date: Fri, 9 May 2025 17:19:16 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Chen Linxuan <chenlinxuan@...ontech.com>
Subject: Re: [PATCH v3 2/3] fs: fuse: add backing_files control file

On Fri, May 9, 2025 at 4:43 PM Chen Linxuan <chenlinxuan@...ontech.com> wrote:
>
> On Fri, May 9, 2025 at 10:00 PM Miklos Szeredi <miklos@...redi.hu> wrote:
> >
> > On Fri, 9 May 2025 at 08:34, Chen Linxuan via B4 Relay
> > <devnull+chenlinxuan.uniontech.com@...nel.org> wrote:
> > >
> > > From: Chen Linxuan <chenlinxuan@...ontech.com>
> > >
> > > Add a new FUSE control file "/sys/fs/fuse/connections/*/backing_files"
> > > that exposes the paths of all backing files currently being used in
> > > FUSE mount points. This is particularly valuable for tracking and
> > > debugging files used in FUSE passthrough mode.
> >
> > This is good work, thanks.
> >
> > My concern is that this is a very fuse specific interface, even though
> > the problem is more generic: list hidden open files belonging to a
> > kernel object, but not installed in any fd:
> >
> >  - SCM_RIGHTS
> >  - io_uring
>
> Note that io_uring has already exposed information about fixed files
> in its fdinfo.
>
> >  - fuse
> >
> > So we could have a new syscall or set of syscalls for this purpose.
> > But that again goes against my "this is not generic enough" pet peeve.
> >
> > So we had this idea of reusing getxattr and listxattr (or implementing
> > a new set of syscalls with the same signature) to allow retrieving a
> > hierarchical set of attributes belonging to a kernel object.  This one
> > would also fit that pattern, so...
> >
> > Thoughts?

I remember that there was some push back on this idea.
If there was no push back, you probably wouldn't have written
listmount/statmount...
so I am a bit concerned about sending Chen down this rabbit hole.

I think that for lsof, any way we present the information in fdinfo
that is parsable would be good enough for lsof to follow.

We could also list a full copy of backing_files table in fdinfo
of all the /dev/fuse open files, that will give lsof the pid of fuse server
in high likelihood.

But this is not very scalable with a large number of backing_files. hmm.

Is it a bad idea to merge the connections/N/backing_files code anyway
at least for debugging?

The extra fdinfo in patch 3 is just useful.
I don't see why we should not add it regardless of the standard  way
to iterate all backing_files.

Thanks,
Amir.