linux-ext4 - Re: [External] Re: [PATCH] block: enable per-cpu bio cache by default

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Message-ID: <CAPFOzZs5mJ9Ts+TYkhioO8aAYfzevcgw7O3hjexFNb_tM+kEZA@mail.gmail.com>
Date: Tue, 14 Oct 2025 10:38:43 +0800
From: Fengnan Chang <changfengnan@...edance.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Christoph Hellwig <hch@...radead.org>, fengnan chang <fengnanchang@...il.com>, axboe@...nel.dk, 
	viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz, 
	willy@...radead.org, djwong@...nel.org, ritesh.list@...il.com, 
	linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org, 
	linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [External] Re: [PATCH] block: enable per-cpu bio cache by default

Pavel Begunkov <asml.silence@...il.com> 于2025年10月13日周一 21:30写道：
>
> On 10/13/25 13:58, Fengnan Chang wrote:
> > Christoph Hellwig <hch@...radead.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.
>
> That can likely be made unconditional as well. Regardless of that,
Agree, IMO we can remove bio_kmalloc in blk_rq_map_bio_alloc, just
use bio_alloc_bioset.  Do this in another patch maybe better ?

> it can't be removed without additional changes because it's used to
> avoid de-allocating into the pcpu cache requests that wasn't
> allocated for it. i.e.
>
> if (bio->bi_opf & REQ_ALLOC_CACHE)
>         bio_put_percpu_cache(bio);
> else
>         bio_free(bio);
>
> Without it under memory pressure you can end up in a situation
> where bios are put into pcpu caches of other CPUs and can't be
> reallocated by the current CPU, effectively loosing the mempool
> forward progress guarantees. See:

Thanks for your remind.

>
> commit 759aa12f19155fe4e4fb4740450b4aa4233b7d9f
> Author: Pavel Begunkov <asml.silence@...il.com>
> Date:   Wed Nov 2 15:18:20 2022 +0000
>
>      bio: don't rob starving biosets of bios
>
> --
> Pavel Begunkov
>