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

[Fengnan Chang <changfengnan@bytedance.com>]

Christoph Hellwig <hch@infradead.org> 于2025年10月13日周一 14:28写道：
>
> On Mon, Oct 13, 2025 at 01:42:47PM +0800, fengnan chang wrote:
> > > 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.
>
> Given that we're using the flag by default and setting it here,
> bio_alloc_clone should not inherit it.  In fact we should probably
> figure out a way to remove it entirely, but if that is not possible
> it should only be set when the cache was actually used.

For now bio_alloc_clone will inherit all flag of source bio, IMO if only not
inherit REQ_ALLOC_CACHE, it's a little strange.
The REQ_ALLOC_CACHE flag can not remove entirely.  maybe we can
modify like this:

if (bs->cache && nr_vecs <= BIO_INLINE_VECS) {
    opf |= REQ_ALLOC_CACHE;
    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;

>
> > > > +     /*
> > > > +      * 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.
>
> So what can't it be deleted?

blk_rq_map_bio_alloc use REQ_ALLOC_CACHE to tell whether to use
bio_alloc_bioset or bio_kmalloc, I considered removing the flag in
blk_rq_map_bio_alloc, but then there would have to be the introduction
of a new flag like  REQ_xx. So I keep this and comment.

>