Re: Why do NBD requests prevent hibernation, and FUSE requests do not?

[Nikolaus Rath <Nikolaus@rath.org>]

On Aug 31 2022, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
> On 8/30/22 08:31, Nikolaus Rath wrote:
>> Hello,
>> I am comparing the behavior of FUSE and NBD when attempting to hibernate
>> the system.
>> FUSE seems to be mostly compatible, I am able to suspend the system even
>> when there is ongoing I/O on the fuse filesystem.
>> 
>
> ....
>
>> As far as I can tell, the problem is that while an NBD request is
>> pending, the atsk that waits for the result (in this case *rsync*) is
>> refusing to freeze. This happens even when setting a 5 minute timeout
>> for freezing (which is more than enough time for the NBD request to
>> complete), so I suspect that the NBD server task (in this case nbdkit)
>> has already been frozen and is thus unable to make progress.
>> However, I do not understand why the same is not happening for FUSE
>> (with FUSE requests being stuck because the FUSE daemon is already
>> frozen). Was I just very lucky in my tests? Or are tasks waiting for
>> FUSE request in a different kind of state? Or is NBD a red-herring here,
>> and the real trouble is with ZFS?
>> It would be great if someone  could shed some light on what's going on.
>
> I guess it is a generic issue also affecting fuse, see this patch
>
> https://lore.kernel.org/lkml/20220511013057.245827-1-dlunev@chromium.org/
>
> A bit down the thread you can find a reference to this ancient patch
>
> https://linux-kernel.vger.kernel.narkive.com/UeBWfN1V/patch-fuse-make-fuse-daemon-frozen-along-with-kernel-threads

Interesting, thank you for the link! So it seems that I just got lucky
with FUSE.

Does anyone know in which order the kernel freezes processes by default?
Could I perhaps work around the problem by calling the FUSE/NBD daemon
something like "zzzzz_mydaemon"?


Best,
-Nikolaus

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