linux-kernel - Re: [fuse-devel] fuse: feasible to distinguish between umount and abort?

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Date:   Tue, 29 Nov 2016 08:01:11 -0800
From:   Nikolaus Rath <Nikolaus@...h.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>
Subject: Re: [fuse-devel] fuse: feasible to distinguish between umount and abort?

On Nov 29 2016, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Fri, Nov 25, 2016 at 1:33 AM, Nikolaus Rath <Nikolaus@...h.org> wrote:
>> On Nov 24 2016, Miklos Szeredi <miklos@...redi.hu> wrote:
>>> On Thu, Nov 24, 2016 at 12:11 AM, Nikolaus Rath <Nikolaus@...h.org> wrote:
>>>> Hello,
>>>>
>>>> Currently, both a call to umount(2) and writing "1" to
>>>> /sys/fs/fuse/connections/NNN/abort will put the /dev/fuse fd into the
>>>> same state: reading from it returns ENODEV, and polling on it returns
>>>> POLLERR.
>>>>
>>>> This causes problems for filesystems that want to ensure that the
>>>> mountpoint is free when they exit. If accessing the device fd gives the
>>>> above errors, they have to do an additional check to determine if they
>>>> still need to unmount the mountpoint. This is difficult to do without
>>>> race conditions (think of someone unmounting and immediately re-starting
>>>> a new filesystem instance).
>>>>
>>>> Would it be possible to change the behavior of the /dev/fuse fd so that
>>>> userspace can distinguish between a regular umount and use of the
>>>> /sys/fs/fuse abort)?
>>>
>>> Yes.  My proposal would be for the kernel to send FUSE_DESTROY
>>> asynchronously and only return ENODEV once that request was read by
>>> userspace.  Currently FUSE_DESTROY is sent synchronously for fuseblk
>>> mounts, but not for plain fuse mounts.
>>
>> I trust that this is a good plan, but from the description I can't quite
>> tell how the filesystem would make the distinction between umount/abort
>> based on this. Would FUSE_DESTROY be send only for unmount, but not for
>> abort?
>
> Right.  The userspace implementation would need to be careful to
> process the DESTROY message before ENODEV received in a different
> thread.   Maybe instead userspace and kernel should negotiate in INIT
> whether userspace wants a DESTROY or not.  If it does, then on umount
> kernel sends DESTROY and does not return ENODEV.  If userspace does
> not want DESTROY then it falls back to the old way of returning
> ENODEV.  And on abort it would do that as well, regardless of the
> negotiated DESTROY request.

That sounds great to me. Would you have to implement this, or should I
try to give it a shot? In the latter case, could you give me a hint
where the entry points for the umount and abort code paths are (in the
kernel code)? 


Best,
-Nikolaus

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