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

Matthew Wilcox
2026-02-26T21:12:22Z

On Wed, Feb 25, 2026 at 08:15:28PM -0700, Jens Axboe wrote:
> On 2/25/26 7:55 PM, Matthew Wilcox wrote:
> > 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?
>
> Perhaps, even though the punting tends to suck... One idea I toyed with
> but had to abandon due to fs freezeing was letting callers that process
> completions in task context anyway just do the necessary work at that
> time. There's literally nothing worse than having part of a completion
> happen in IRQ, then punt parts of that to a worker, and need to wait for
> the worker to finish whatever it needs to do - only to then wake the
> target task. We can trivially do this in io_uring, as the actual
> completion is posted from the task itself anyway. We just need to have
> the task do the bottom half of the completion as well, rather than some
> unrelated kthread worker.
>
> I'd be worried a generic solution would be the worst of all worlds, as
> it prevents optimizations that happen in eg iomap and other spots, where
> only completions that absolutely need to happen in task context get
> punted. There's a big difference between handling a completion inline vs
> needing a round-trip to some worker to do it.

I spoke a little hastily when I said "all write completions". What I
really meant was something like:

+++ b/block/bio.c
@@ -1788,7 +1788,9 @@ void bio_endio(struct bio *bio)
}
#endif

- if (bio->bi_end_io)
+ if (!in_task() && bio_flagged(bio, BIO_COMPLETE_IN_TASK_CONTEXT))
+ bio_queue_completion(bio);
+ else if (bio->bi_end_io)
bio->bi_end_io(bio);
}
EXPORT_SYMBOL(bio_endio);

and then the submitter (ie writeback) would choose to set
BIO_COMPLETE_IN_TASK_CONTEXT. And maybe others (eg fscrypt) would
want to do the same.