Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Filesystem Suspend Resume

Dave Chinner
2025-03-24T20:50:26Z

On Mon, Mar 24, 2025 at 12:38:20PM +0100, Jan Kara wrote:
> On Fri 21-03-25 13:00:24, James Bottomley via Lsf-pc wrote:
> > On Fri, 2025-03-21 at 08:34 -0400, James Bottomley wrote:
> > [...]
> > > Let me digest all that and see if we have more hope this time around.
> >
> > OK, I think I've gone over it all. The biggest problem with
> > resurrecting the patch was bugs in ext3, which isn't a problem now.
> > Most of the suspend system has been rearchitected to separate
> > suspending user space processes from kernel ones. The sync it
> > currently does occurs before even user processes are frozen. I think
> > (as most of the original proposals did) that we just do freeze all
> > supers (using the reverse list) after user processes are frozen but
> > just before kernel threads are (this shouldn't perturb the image
> > allocation in hibernate, which was another source of bugs in xfs).
>
> So as far as my memory serves the fundamental problem with this approach
> was FUSE - once userspace is frozen, you cannot write to FUSE filesystems
> so filesystem freezing of FUSE would block if userspace is already
> suspended. You may even have a setup like:
>
> bdev <- fs <- FUSE filesystem <- loopback file <- loop device <- another fs
>
> So you really have to be careful to freeze this stack without causing
> deadlocks. So you need to be freezing userspace after filesystems are
> frozen but then you have to deal with the fact that parts of your userspace
> will be blocked in the kernel (trying to do some write) waiting for the
> filesystem to thaw. But it might be tractable these days since I have a
> vague recollection that system suspend is now able to gracefully handle
> even tasks in uninterruptible sleep.

I thought we largely solved this problem with userspace flusher
threads being able to call prctl(PR_IO_FLUSHER) to tell the kernel
they are part of the IO stack and so need to be considered
special from the POV of memory allocation and write (dirty page)
throttling.

Maybe hibernate needs to be aware of these userspace flusher
tasks and only suspend them after filesystems are frozen instead
of when userspace is initially halted?

-Dave.
--
Dave Chinner
david@fromorbit.com