linux-kernel - Re: [PATCH v3 0/3] fuse: Expose more information of fuse backing files to userspace

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Message-ID: <CAJnrk1ZxpOBENHk3Q1dJVY78RSdE+PtFR8UpYukT0dLJv3scHw@mail.gmail.com>
Date: Tue, 13 May 2025 11:52:20 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Amir Goldstein <amir73il@...il.com>, chenlinxuan@...ontech.com, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Bernd Schubert <bernd.schubert@...tmail.fm>
Subject: Re: [PATCH v3 0/3] fuse: Expose more information of fuse backing
 files to userspace

On Fri, May 9, 2025 at 6:23 AM Miklos Szeredi <miklos@...redi.hu> wrote:
>
> On Fri, 9 May 2025 at 09:36, Amir Goldstein <amir73il@...il.com> wrote:
>
> > This is not the case with displaying conn, because lsof is not designed
> > to list fuse conn.
> >
> > Is there a way for userspace to get from conn to fuse server pid?
>
> Define "server pid".
>
> One such definition would be
>
>  "A fuse server is a process that has an open file descriptor
> referring to a /dev/fuse instance."
>
> This definition allows a mapping between tasks and fuse connections to
> be established.  Note that this is not a 1:1 mapping.  Multiple
> processes could have the same fuse fd (or a clone) open and one
> process could have multiple different connections associated with it.
>
> This might be sufficient for lsof if it can find out the connection
> number from the fd.  E.g. adding "fuse_connection: N" to fdinfo would
> work, I think.

For getting from conn to fuse server pid, what about adding the server
pid to fuse's sysfs info? This use case has come up a few times in
production where we've encountered a stuck server and have wanted to
identify its pid. I have a patch on a branch I need to clean up and
send out for this, but it adds a new "info" file to
/sys/fs/fuse/connections/*/ where libfuse can write any identifying
info to that file like the server pid or name. If the connection gets
migrated to another process then libfuse is responsible for modifying
that to reflect the correct info.

Thanks,
Joanne

>
> Thanks,
> Miklos
>