Re: [PATCH RFC v2 1/2] filemap: defer dropbehind invalidation from IRQ context

Matthew Wilcox
2026-02-26T02:55:35Z

On Wed, Feb 25, 2026 at 03:52:41PM -0700, Jens Axboe wrote:
> How well does this scale? I did a patch basically the same as this, but
> not using a folio batch though. But the main sticking point was
> dropbehind_lock contention, to the point where I left it alone and
> thought "ok maybe we just do this when we're done with the awful
> buffer_head stuff". What happens if you have N threads doing IO at the
> same time to N block devices? I suspect it'll look absolutely terrible,
> as each thread will be banging on that dropbehind_lock.
>
> One solution could potentially be to use per-cpu lists for this. If you
> have N threads working on separate block devices, they will tend to be
> sticky to their CPU anyway.

Back in 2021, I had Vishal look at switching the page cache from using
hardirq-disabling locks to softirq-disabling locks [1]. Some of the
feedback (which doesn't seem to be entirely findable on the lists ...)
was that we'd be better off punting writeback completion from interrupt
context to task context and going from spin_lock_irq() to spin_lock()
rather than going to spin_lock_bh().

I recently saw something (possibly XFS?) promoting this idea again.
And now there's this. Perhaps the time has come to process all
write-completions in task context, rather than everyone coming up with
their own workqueues to solve their little piece of the problem?

[1] https://lore.kernel.org/linux-block/20210730213630.44891-1-vishal.moola@gmail.com/