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

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Message-ID: <aOyb-NyCopUKridK@infradead.org>
Date: Sun, 12 Oct 2025 23:28:08 -0700
From: Christoph Hellwig <hch@...radead.org>
To: fengnan chang <fengnanchang@...il.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Fengnan Chang <changfengnan@...edance.com>, axboe@...nel.dk,
	viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	asml.silence@...il.com, 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: [PATCH] block: enable per-cpu bio cache by default

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.

> > > +     /*
> > > +      * 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?