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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Matthew Wilcox <willy@infradead.org> writes:

> 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?

I am hoping the author of this patch or folks with io-uring expertise
(which added the per-cpu bio cache in the first place) could answer
this better. i.e. 

Now that per-cpu bio cache is being used by io-uring rw requests for
both polled and non-polled I/O. Does that mean, we can kill
IOCB_ALLOC_CACHE check from iomap dio path completely and use per-cpu
bio cache unconditionally by passing REQ_ALLOC_CACHE flag?  That means
all DIO requests via iomap can now use this per-cpu bio cache and not
just the one initiated via io-uring path.

Or are there still restrictions in using this per-cpu bio cache, which
limits it to be only used via io-uring path? If yes, what are they? And
can this be documented somewhere?

-ritesh