linux-kernel - Re: [Drbd-dev] [PATCH v3 06/26] block: Add bio_end

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Date:	Tue, 25 Sep 2012 13:54:52 +0200
From:	Lars Ellenberg <lars.ellenberg@...bit.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, axboe@...nel.dk, linux-s390@...r.kernel.org,
	Chris Mason <chris.mason@...ionio.com>, neilb@...e.de,
	Jiri Kosina <jkosina@...e.cz>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Alasdair Kergon <agk@...hat.com>, tj@...nel.org,
	Steven Whitehouse <swhiteho@...hat.com>, vgoyal@...hat.com,
	Lars Ellenberg <drbd-dev@...ts.linbit.com>
Subject: Re: [Drbd-dev] [PATCH v3 06/26] block: Add bio_end_sector()

On Mon, Sep 24, 2012 at 03:34:46PM -0700, Kent Overstreet wrote:
> Just a little convenience macro - main reason to add it now is preparing
> for immutable bio vecs, it'll reduce the size of the patch that puts
> bi_sector/bi_size/bi_idx into a struct bvec_iter.


For the DRBD part:

> diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c
> index 01b2ac6..d90a1fd 100644
> --- a/drivers/block/drbd/drbd_req.c
> +++ b/drivers/block/drbd/drbd_req.c
> @@ -1144,7 +1144,7 @@ void drbd_make_request(struct request_queue *q, struct bio *bio)
>  	/* to make some things easier, force alignment of requests within the
>  	 * granularity of our hash tables */
>  	s_enr = bio->bi_sector >> HT_SHIFT;
> -	e_enr = bio->bi_size ? (bio->bi_sector+(bio->bi_size>>9)-1) >> HT_SHIFT : s_enr;
> +	e_enr = (bio_end_sector(bio) - 1) >> HT_SHIFT;

You ignored the bio->bi_size ? : ;

#define bio_end_sector(bio)	((bio)->bi_sector + bio_sectors(bio))
which turns out (bio->bi_sector + (bio->bi_size >> 9))

Note that bi_size may be 0, bio_end_sector(bio)-1 then is bi_sector -1,
for an empty flush with bi_sector == 0, this ends up as (sector_t)-1ULL,
and this code path breaks horribly.


+	e_enr = bio->bi_size ? (bio_end_sector(bio) - 1) >> HT_SHIFT : s_enr;


Thanks,
	Lars
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