Re: [fuse-devel] [PATCH] fuse: make fuse daemon frozen along with kernel threads

[Miklos Szeredi <miklos@szeredi.hu>]

[CC list restored]

On Thu, Feb 7, 2013 at 9:41 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> On Wed, Feb 06, 2013 at 10:27:40AM +0100, Miklos Szeredi wrote:
>> On Wed, Feb 6, 2013 at 2:11 AM, Li Fei <fei.li@intel.com> wrote:
>> >
>> > There is well known issue that freezing will fail in case that fuse
>> > daemon is frozen firstly with some requests not handled, as the fuse
>> > usage task is waiting for the response from fuse daemon and can't be
>> > frozen.
>> >
>> > To solve the issue above, make fuse daemon frozen after all all user
>> > space processes frozen and during the kernel threads frozen phase.
>> > PF_FREEZE_DAEMON flag is added to indicate that the current thread is
>> > the fuse daemon,
>>
>> Okay and how do you know which thread, process or processes belong to
>> the "fuse daemon"?
>
> Maybe I'm talking about the wrong thing but isn't any process having
> /dev/fuse open "the fuse daemon"? And that doesn't even cover cases
> where one thread reads requests from the kernel and hands them to
> worker threads (that do not have /dev/fuse themself). Or the fuse
> request might need mysql to finish a request.
>
> I believe figuring out what processes handle fuse requests is a lost
> proposition.

Pretty much.

>
>
> Secondly how does freezing the daemon second garanty that it has
> replied to all pending requests? Or how is leaving it thawed the right
> decision?
>
> Instead the kernel side of fuse should be half frozen and stop sending
> out new requests. Then it should wait for all pending requests to
> complete. Then and only then can userspace processes be frozen safely.

The problem with that is one fuse filesystem might be calling into
another.  Say two fuse filesystems are mounted at /mnt/A and /mnt/B,
Process X starts a read request on /mnt/A. This is handled by process
A, which in turn starts a read request on /mnt/B, which is handled by
B.  If we freeze the system at the moment when A starts handling the
request but hasn't yet sent the request to B then things wil be stuck
and the freezing will fail.

So the order should be:  Freeze the "topmost" fuse filesystems (A in
the example) first and if all pending requests are completed then
freeze the next ones in the hierarchy, etc...  This would work if this
dependency between filesystems were known.  But it's not and again it
looks like an impossible task.

The only way to *reliably* freeze fuse filesystems is to let it freeze
even if there are outstanding requests.  But that's the hardest to
implement, because then it needs to allow freezing of tasks waiting on
i_mutex, for example, which is currently not possible.  But this is
the only way I see that would not have unsolvable corner cases that
prop up at the worst moment.

And yes, it would be prudent to  wait some time for pending requests
to finish before freezing.  But it's not a requirement, things
*should* work without that: suspending a machine is just like a very
long pause by the CPU, as long as no hardware is involved.  And with
fuse filesystems there is no hardware involved directly by definition.

But I'm open to ideas and at this stage I think even patches that
improve the situation for the majority of the cases would be
acceptable, since this is turning out to be a major problem for a lot
of people.

Thanks,
Miklos