Re: [PATCH] loop: add WQ_MEM_RECLAIM flag to per device workqueue

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, Mar 22, 2022 at 09:09:53AM +0900, Tetsuo Handa wrote:
> > The legacy flushing warning is telling us that those workqueues can be
> 
> s/can be/must be/ ?

Well, one thing that we can but don't want to do is converting all
create_workqueue() users to alloc_workqueue() with MEM_RECLAIM, which is
wasteful but won't break anything. We know for sure that the workqueues
which trigger the legacy warning are participating in memory reclaim and
thus need MEM_RECLAIM. So, yeah, the warning tells us that they need
MEM_RECLAIM and should be converted.

> ? Current /* internal: create*_workqueue() */ tells me nothing.

It's trying to say that it shouldn't be used outside workqueue proper and
the warning message is supposed to trigger the conversion. But, yeah, a
stronger comment can help.

> My question is: I want to add WQ_MEM_RECLAIM flag to the WQ used by loop module
> because this WQ is involved upon writeback operation. But unless I add both
> __WQ_LEGACY | WQ_MEM_RECLAIM flags to the WQ used by loop module, we will hit
> 
> 	WARN_ONCE(worker && ((worker->current_pwq->wq->flags &
> 			      (WQ_MEM_RECLAIM | __WQ_LEGACY)) == WQ_MEM_RECLAIM),
> 
> warning because e.g. xfs-sync WQ used by xfs module is not using WQ_MEM_RECLAIM flag.
> 
> 	mp->m_sync_workqueue = alloc_workqueue("xfs-sync/%s",
> 				XFS_WQFLAGS(WQ_FREEZABLE), 0, mp->m_super->s_id);
> 
> You are suggesting that the correct approach is to add WQ_MEM_RECLAIM flag to WQs
> used by filesystems when adding WQ_MEM_RECLAIM flag to the WQ used by loop module
> introduces possibility of hitting
> 
> 	WARN_ONCE(worker && ((worker->current_pwq->wq->flags &
> 			      (WQ_MEM_RECLAIM | __WQ_LEGACY)) == WQ_MEM_RECLAIM),
> 
> warning (instead of either adding __WQ_LEGACY | WQ_MEM_RECLAIM flags to the WQ used
> by loop module or giving up WQ_MEM_RECLAIM flag for the WQ used by loop module),
> correct?

Yeah, you detected multiple issues at the same time. xfs sync is
participating in memory reclaim but doesn't have MEM_RECLAIM and loop is
marked with LEGACY but flushing other workqueues which are MEM_RECLIAM. So,
both xfs and loop workqueues need to be explicitly marked with MEM_RECLAIM.

Thanks.

-- 
tejun