Re: [RFC PATCH] Page Cache Allowing Hard Interrupts

[Christoph Hellwig]

On Tue, Aug 10, 2021 at 01:09:45PM +0100, Matthew Wilcox wrote:
> On Tue, Aug 10, 2021 at 09:15:28AM +0100, Christoph Hellwig wrote:
> > Stupid question, but where do we ever do page cache interaction from
> > soft irq context?
> 
> test_clear_page_writeback() happens in _some_ interrupt context (ie
> the io completion path).  We had been under the impression that it was
> always actually softirq context, and so this patch was safe.  However,
> it's now clear that some drivers are calling it from hardirq context.
> Writeback completions are clearly not latency sensitive and so can
> be delayed from hardirq to softirq context without any problem, so I
> think fixing this is just going to be a matter of tagging requests as
> "complete in softirq context" and ensuring that blk_mq_raise_softirq()
> is called for them.
> 
> Assuming that DIO write completions _are_ latency-sensitive, of course.
> Maybe all write completions could be run in softirqs.

I really don't really see any benefit in introducing softirqs into
the game.  If we want to simplify the locking and do not care too much
about latency, we should just defer to workqueue/thread context.
For example XFS already does that for all writeback except for pure
overwrites.  Those OTOH can be latency critical for O_SYNC writes, but
you're apparently looking into that already.