Re: [PATCH v2 07/26] block: Don't use bi_idx in bio_split() or require it to be 0

[Kent Overstreet <koverstreet@google.com>]

On Thu, Sep 20, 2012 at 04:45:44PM -0700, Tejun Heo wrote:
> On Mon, Sep 10, 2012 at 05:22:18PM -0700, Kent Overstreet wrote:
> > Prep work for immutable bio_vecs/efficient bio splitting: they require
> > auditing and removing most uses of bi_idx.
> > 
> > So here we convert bio_split() to respect the current value of bi_idx
> > and use the bio_iovec() macro, instead of assuming bi_idx will be 0.
> 
> I find the description a bit cryptic.

Yeah, I wasn't able to come up with a better description at the time...
how's this:

Change bio_split() to respect the current value of bi_idx

In the current code bio_split() won't be seeing partially completed bios
so this doesn't change any behaviour, but this makes the code a bit
clearer as to what bio_split() actually requires.

The immediate purpose of the patch is removing unnecessary bi_idx
references, but the end goal is to allow partial completed bios to be
submitted, which along with immutable biovecs enables effecient bio
splitting.


> 
> > Signed-off-by: Kent Overstreet <koverstreet@google.com>
> > CC: Jens Axboe <axboe@kernel.dk>
> > ---
> >  drivers/block/drbd/drbd_req.c | 6 +++---
> >  drivers/md/raid0.c            | 3 +--
> >  drivers/md/raid10.c           | 3 +--
> >  fs/bio-integrity.c            | 4 ++--
> >  fs/bio.c                      | 7 +++----
> >  5 files changed, 10 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c
> > index af69a96..57eb253 100644
> > --- a/drivers/block/drbd/drbd_req.c
> > +++ b/drivers/block/drbd/drbd_req.c
> > @@ -1155,11 +1155,11 @@ void drbd_make_request(struct request_queue *q, struct bio *bio)
> >  
> >  	/* can this bio be split generically?
> >  	 * Maybe add our own split-arbitrary-bios function. */
> > -	if (bio->bi_vcnt != 1 || bio->bi_idx != 0 || bio->bi_size > DRBD_MAX_BIO_SIZE) {
> > +	if (bio_segments(bio) != 1 || bio->bi_size > DRBD_MAX_BIO_SIZE) {
> >  		/* rather error out here than BUG in bio_split */
> >  		dev_err(DEV, "bio would need to, but cannot, be split: "
> > -		    "(vcnt=%u,idx=%u,size=%u,sector=%llu)\n",
> > -		    bio->bi_vcnt, bio->bi_idx, bio->bi_size,
> > +		    "(segments=%u,size=%u,sector=%llu)\n",
> > +		    bio_segments(bio), bio->bi_size,
> >  		    (unsigned long long)bio->bi_sector);
> >  		bio_endio(bio, -EINVAL);
> >  	} else {
> > diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
> > index 387cb89..0587450 100644
> > --- a/drivers/md/raid0.c
> > +++ b/drivers/md/raid0.c
> > @@ -509,8 +509,7 @@ static void raid0_make_request(struct mddev *mddev, struct bio *bio)
> >  		sector_t sector = bio->bi_sector;
> >  		struct bio_pair *bp;
> >  		/* Sanity check -- queue functions should prevent this happening */
> > -		if (bio->bi_vcnt != 1 ||
> > -		    bio->bi_idx != 0)
> > +		if (bio_segments(bio) != 1)
> >  			goto bad_map;
> >  		/* This is a one page bio that upper layers
> >  		 * refuse to split for us, so we need to split it.
> > diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> > index 9715aaf..bbd08f5 100644
> > --- a/drivers/md/raid10.c
> > +++ b/drivers/md/raid10.c
> > @@ -1081,8 +1081,7 @@ static void make_request(struct mddev *mddev, struct bio * bio)
> >  			 || conf->prev.near_copies < conf->prev.raid_disks))) {
> >  		struct bio_pair *bp;
> >  		/* Sanity check -- queue functions should prevent this happening */
> > -		if (bio->bi_vcnt != 1 ||
> > -		    bio->bi_idx != 0)
> > +		if (bio_segments(bio) != 1)
> >  			goto bad_map;
> >  		/* This is a one page bio that upper layers
> >  		 * refuse to split for us, so we need to split it.
> 
> And wonder how the description applies to the above.
> 
> > --- a/fs/bio.c
> > +++ b/fs/bio.c
> > @@ -1616,8 +1616,7 @@ struct bio_pair *bio_split(struct bio *bi, int first_sectors)
> >  	trace_block_split(bdev_get_queue(bi->bi_bdev), bi,
> >  				bi->bi_sector + first_sectors);
> >  
> > -	BUG_ON(bi->bi_vcnt != 1);
> > -	BUG_ON(bi->bi_idx != 0);
> > +	BUG_ON(bio_segments(bi) != 1);
> >  	atomic_set(&bp->cnt, 3);
> >  	bp->error = 0;
> >  	bp->bio1 = *bi;
> > @@ -1626,8 +1625,8 @@ struct bio_pair *bio_split(struct bio *bi, int first_sectors)
> >  	bp->bio2.bi_size -= first_sectors << 9;
> >  	bp->bio1.bi_size = first_sectors << 9;
> >  
> > -	bp->bv1 = bi->bi_io_vec[0];
> > -	bp->bv2 = bi->bi_io_vec[0];
> > +	bp->bv1 = *bio_iovec(bi);
> > +	bp->bv2 = *bio_iovec(bi);
> 
> This conflicts with a recent commit from Martin.  You probably wanna
> rebase.
> 
> Thanks.
> 
> -- 
> tejun