linux-kernel - Re: [PATCH 20/27] dm: dm-bufio.c: use bio_set_vec

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Message-ID: <20160405181123.GA18323@redhat.com>
Date:	Tue, 5 Apr 2016 14:11:23 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Ming Lei <tom.leiming@...il.com>, Jens Axboe <axboe@...com>,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	Boaz Harrosh <boaz@...xistor.com>,
	Alasdair Kergon <agk@...hat.com>,
	"maintainer:DEVICE-MAPPER (LVM)" <dm-devel@...hat.com>,
	Shaohua Li <shli@...nel.org>,
	"open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
	<linux-raid@...r.kernel.org>
Subject: Re: [PATCH 20/27] dm: dm-bufio.c: use bio_set_vec_table()

On Tue, Apr 05 2016 at  9:04am -0400,
Christoph Hellwig <hch@...radead.org> wrote:

> On Tue, Apr 05, 2016 at 08:07:35PM +0800, Ming Lei wrote:
> > Signed-off-by: Ming Lei <tom.leiming@...il.com>
> > ---
> >  drivers/md/dm-bufio.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> > index cd77216..0e48ad7 100644
> > --- a/drivers/md/dm-bufio.c
> > +++ b/drivers/md/dm-bufio.c
> > @@ -624,8 +624,7 @@ static void use_inline_bio(struct dm_buffer *b, int rw, sector_t block,
> >  	int len;
> >  
> >  	bio_init(&b->bio);
> > -	b->bio.bi_io_vec = b->bio_vec;
> > -	b->bio.bi_max_vecs = DM_BUFIO_INLINE_VECS;
> > +	bio_set_vec_table(&b->bio, b->bio_vec, DM_BUFIO_INLINE_VECS);
> >  	b->bio.bi_iter.bi_sector = block << b->c->sectors_per_block_bits;
> >  	b->bio.bi_bdev = b->c->bdev;
> >  	b->bio.bi_end_io = inline_endio;
> 
> Should be switched to use bio_alloc instead.

Why does the use of bio_init() vs bio_alloc() bother you?

'struct dm_buffer' has a 'struct bio'.  That bio is allocated as part of
the dm_buffer in drivers/md/dmbufio.c:alloc_buffer() -- which is called
by various other bufio interfaces (e.g. __alloc_buffer_wait).  Bufio is
in control of ensuring forward progress by carefully managing the memory
associated with these buffers.  I don't see the benefit of bio_alloc()
here.  What am I missing?