Re: [PATCH] block: enable per-cpu bio cache by default

[fengnan chang <fengnanchang@gmail.com>]

On Mon, Oct 13, 2025 at 11:25 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Sat, Oct 11, 2025 at 09:33:12AM +0800, Fengnan Chang wrote:
> > +     opf |= REQ_ALLOC_CACHE;
> > +     if (bs->cache && nr_vecs <= BIO_INLINE_VECS) {
> > +             bio = bio_alloc_percpu_cache(bdev, nr_vecs, opf,
> > +                                          gfp_mask, bs);
> > +             if (bio)
> > +                     return bio;
> > +             /*
> > +              * No cached bio available, bio returned below marked with
> > +              * REQ_ALLOC_CACHE to participate in per-cpu alloc cache.
> > +              */
> > +     } else
>
> > +             opf &= ~REQ_ALLOC_CACHE;
>
> Just set the req flag in the branch instead of unconditionally setting
> it and then clearing it.

clearing this flag is necessary, because bio_alloc_clone will call this in
boot stage, maybe the bs->cache of the new bio is not initialized yet.

>
> > +     /*
> > +      * Even REQ_ALLOC_CACHE is enabled by default, we still need this to
> > +      * mark bio is allocated by bio_alloc_bioset.
> > +      */
> >       if (rq->cmd_flags & REQ_ALLOC_CACHE && (nr_vecs <= BIO_INLINE_VECS)) {
>
> I can't really parse the comment, can you explain what you mean?

This is to tell others that REQ_ALLOC_CACHE can't be deleted here, and
that this flag
serves other purposes here.

>
>