linux-kernel - Re: [PATCH v2 2/2] FUSE: Retire superblock on force unmount

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Date:   Wed, 18 May 2022 13:57:45 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Daniil Lunev <dlunev@...omium.org>, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>,
        "Theodore Ts'o" <tytso@....edu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] FUSE: Retire superblock on force unmount

On Wed, 18 May 2022 at 00:32, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Tue, May 17, 2022 at 10:20:06PM +0000, Al Viro wrote:
> > On Thu, May 12, 2022 at 08:29:10AM +1000, Daniil Lunev wrote:
> > > Force unmount of FUSE severes the connection with the user space, even
> > > if there are still open files. Subsequent remount tries to re-use the
> > > superblock held by the open files, which is meaningless in the FUSE case
> > > after disconnect - reused super block doesn't have userspace counterpart
> > > attached to it and is incapable of doing any IO.
> >
> >       Why not simply have those simply rejected by fuse_test_super()?
> > Looks like that would be much smaller and less invasive patch...
> > Confused...
>
> ... because Miklos had suggested that, apparently ;-/  I disagree -
> that approach has more side effects.  "mount will skip that sucker" is,
> AFAICS, the only effect of modiyfing test_super callback(s); yours, OTOH...

Yep, messing with the bdi doesn't look good.  Fuse always uses a
private bdi, so it's not even necessary.

Just removing from type->fs_supers should not have any side effects,
at least I can't spot any.

Fixing fuse_test_super() is not sufficient, as the fuseblk type goes
though get_tree_bdev().  That could be tweaked as well, but it would
end up with more complexity.

Thanks,
Miklos