linux-kernel - Re: [PATCH V7 20/24] bcache: avoid to use bio_for_each_segment_all() in bch_bio_alloc

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Date:   Thu, 28 Jun 2018 09:28:18 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Coly Li <colyli@...e.de>
Cc:     Jens Axboe <axboe@...com>, Christoph Hellwig <hch@...radead.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        David Sterba <dsterba@...e.cz>,
        Huang Ying <ying.huang@...el.com>,
        Mike Snitzer <snitzer@...hat.com>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Theodore Ts'o <tytso@....edu>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Filipe Manana <fdmanana@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-bcache@...r.kernel.org
Subject: Re: [PATCH V7 20/24] bcache: avoid to use bio_for_each_segment_all()
 in bch_bio_alloc_pages()

On Wed, Jun 27, 2018 at 11:55:33PM +0800, Coly Li wrote:
> On 2018/6/27 8:45 PM, Ming Lei wrote:
> > bch_bio_alloc_pages() is always called on one new bio, so it is safe
> > to access the bvec table directly. Given it is the only kind of this
> > case, open code the bvec table access since bio_for_each_segment_all()
> > will be changed to support for iterating over multipage bvec.
> > 
> > Cc: Coly Li <colyli@...e.de>
> > Cc: linux-bcache@...r.kernel.org
> > Signed-off-by: Ming Lei <ming.lei@...hat.com>
> > ---
> >  drivers/md/bcache/util.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/md/bcache/util.c b/drivers/md/bcache/util.c
> > index fc479b026d6d..9f2a6fd5dfc9 100644
> > --- a/drivers/md/bcache/util.c
> > +++ b/drivers/md/bcache/util.c
> > @@ -268,7 +268,7 @@ int bch_bio_alloc_pages(struct bio *bio, gfp_t gfp_mask)
> >  	int i;
> >  	struct bio_vec *bv;
> > 
> 
> Hi Ming,
> 
> > -	bio_for_each_segment_all(bv, bio, i) {
> > +	for (i = 0, bv = bio->bi_io_vec; i < bio->bi_vcnt; bv++) {
> 
> 
> Is it possible to treat this as a special condition of
> bio_for_each_segement_all() ? I mean only iterate one time in
> bvec_for_each_segment(). I hope the above change is not our last choice
> before I reply an Acked-by :-)

Now the bvec from bio_for_each_segement_all() can't be changed any more
since the referenced 'bvec' is generated in-flight given we store
real multipage bvec.

BTW, this way is actually suggested by Christoph for saving one new
helper of bio_for_each_bvec_all() as done in V6, and per previous discussion,
seems both Kent and Christoph agrees to convert bcache into bio_add_page()
finally.

So I guess this open code style should be fine.

Thanks,
Ming