Re: [PATCH] iomap: allow iomap using the per-cpu bio cache

[Matthew Wilcox <willy@infradead.org>]

On Fri, Aug 22, 2025 at 09:37:32PM +0530, Ritesh Harjani wrote:
> Matthew Wilcox <willy@infradead.org> writes:
> > On Fri, Aug 22, 2025 at 08:05:50AM -0700, Darrick J. Wong wrote:
> >> Is there a reason /not/ to use the per-cpu bio cache unconditionally?
> >
> > AIUI it's not safe because completions might happen on a different CPU
> > from the submission.
> 
> At max the bio de-queued from cpu X can be returned to cpu Y cache, this
> shouldn't be unsafe right? e.g. bio_put_percpu_cache(). 
> Not optimal for performance though.
> 
> Also even for io-uring the IRQ completions (non-polling requests) can
> get routed to a different cpu then the submitting cpu, correct?
> Then the completions (bio completion processing) are handled via IPIs on
> the submtting cpu or based on the cache topology, right?
> 
> > At least, there's nowhere that sets REQ_ALLOC_CACHE unconditionally.
> >
> > This could do with some better documentation ..
> 
> Agreed. Looking at the history this got added for polling mode first but
> later got enabled for even irq driven io-uring rw requests [1]. So it
> make sense to understand if this can be added unconditionally for DIO
> requests or not.

So why does the flag now exist at all?  Why not use the cache
unconditionally?