Re: [PATCH v2] f2fs: quiesce background threads during system suspend using PM notifier

[Bart Van Assche <bvanassche@acm.org>]

On 8/10/26 8:54 AM, Christoph Hellwig wrote:
> On Thu, Aug 06, 2026 at 10:01:08AM -0700, Daeho Jeong wrote:
>> From: Daeho Jeong <daehojeong@google.com>
>>
>> During system suspend, a race condition can cause f2fs_gc and f2fs_discard
>> threads to call submit_bio() while the underlying block device (e.g., UFS)
>> is in Runtime PM suspend. Because Runtime PM worker threads are already
>> frozen during task freezing, the threads become trapped in
>> __bio_queue_enter() waiting on mq_freeze_wq, leading to a PM freezer
>> timeout.
> 
> That does sound like a general issue with our block device / threading
> handling.

Daeho's description above mixes up unrelated topics. A runtime suspended 
UFS device is resumed automatically by the block layer if necessary. The
issue Daeho is trying to solve is unrelated to runtime suspend according
to my understanding.

>> To prevent this deadlock, register a PM notifier to set SBI_IS_SUSPENDING
>> during PM_SUSPEND_PREPARE. Background GC and discard threads check this
>> flag and immediately stop issuing new bios, allowing them to enter a
>> freezable sleep state cleanly before process freezing begins.
>>
>> In addition, check freezing() as a fast path to stop issuing new I/O
>> when non-PM freezing (e.g. dm-snapshot or cgroup freezer) is requested.
> 
> .. which means that we really sould have all the relevant parties
> invited into figuring out whast is happening here, rather than
> band-aiding something that looks like a horrible hack inside a
> file system.
> 
> Unfortunately I see this a lot with f2fs.  Please reach out to all
> relevant maintainers for something that does not look strictly local
> to f2fs.

This information was shared earlier with Daeho 
(https://b.corp.google.com/issues/515470309#comment45): [ ... ]
Register a Freezable Kernel Thread (Recommended) [ ... ]
Why this works: During suspend, freezer stops freezable kernel threads 
before devices and block queues enter their PM suspend phases. When the 
queue freezes later, your thread is already safely sleeping in 
try_to_freeze() and will not attempt submit_bio(). [ ... ]

PS: I'm no longer subscribed to the f2fs-devel mailing list.

Bart.