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

Jan Kara
2025-03-26T14:59:38Z

On Tue 25-03-25 22:36:56, James Bottomley wrote:
> On Tue, 2025-03-25 at 14:42 +0100, Jan Kara wrote:
> [...]
> > If I remember correctly, the problem in the past was, that if you
> > leave userspace running while freezing filesystems, some processes
> > may enter uninterruptible sleep waiting for fs to be thawed and in
> > the past suspend code was not able to hibernate such processes. But I
> > think this obstacle has been removed couple of years ago as now we
> > could use TASK_FREEZABLE flag in sb_start_write() ->
> > percpu_rwsem_wait and thus allow tasks blocked on frozen filesystem
> > to be hibernated.
>
> I tested this and we do indeed deadlock hibernation on the processes
> touching the filesystem (systemd-journald actually). But if I make
> this change:
>
> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
> index 6083883c4fe0..720418720bbc 100644
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -156,7 +156,7 @@ static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
> spin_unlock_irq(&sem->waiters.lock);
>
> while (wait) {
> - set_current_state(TASK_UNINTERRUPTIBLE);
> + set_current_state(TASK_UNINTERRUPTIBLE|TASK_FREEZABLE);
> if (!smp_load_acquire(&wq_entry.private))
> break;
> schedule();
>
> Then everything will work, with no lockdep problems (thanks,
> Christian). Is that the change you want me to make or should
> sb_start_write be using a special freezable version of
> percpu_rwsem_wait()?

I was thinking about this. The possible problem with this may be that a
task waiting in percpu_rwsem_wait() is hibernated and if it holds another
lock (e.g. some mutex) and there's another task waiting for this mutex,
then hibernation fails because that other task cannot be hibernated. With
sb_start_write() specifically, this is usually not a problem because this
is the outermoust lock we take. The only catch here would be if a process
is blocked in a write page fault for a frozen filesystem. Then we are
holding mmap_sem for the process so hibernation could fail this way. But
I'd guess this is rare enough that we could live with that possibility.

So to summarize I think we may need to introduce freezable variant of
percpu_rwsem_down_read() and use it in sb_start_write().

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR