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Message-ID: <20120822210410.GL19212@google.com>
Date: Wed, 22 Aug 2012 14:04:10 -0700
From: Tejun Heo <tj@...nel.org>
To: Kent Overstreet <koverstreet@...gle.com>
Cc: linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
dm-devel@...hat.com, vgoyal@...hat.com, mpatocka@...hat.com,
bharrosh@...asas.com, Jens Axboe <axboe@...nel.dk>,
NeilBrown <neilb@...e.de>,
Lars Ellenberg <lars.ellenberg@...bit.com>,
Peter Osterlund <petero2@...ia.com>,
Sage Weil <sage@...tank.com>,
"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v6 11/13] block: Rework bio_pair_split()
Hello, Kent.
On Wed, Aug 22, 2012 at 10:04:08AM -0700, Kent Overstreet wrote:
> This changes bio_pair_split() to use the new bio_split() underneath,
> which gets rid of the single page bio limitation. The various callers
> are fixed up for the slightly different struct bio_pair, and to remove
> the unnecessary checks.
This changes an existing API both in its interface and behavior and
there's no detailed explanation on how it's changed and what are the
implications.
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index 1f5b483..63e5852 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -751,14 +751,13 @@ static struct bio *bio_chain_clone(struct bio **old, struct bio **next,
>
> /* split the bio. We'll release it either in the next
> call, or it will have to be released outside */
> - bp = bio_pair_split(old_chain,
> - (len - total) / SECTOR_SIZE);
> + bp = bio_pair_split(old_chain, (len - total) / SECTOR_SIZE);
Probably belongs to the previous patch which renamed bio_split() to
bio_pair_split()? Another thing is, is this renaming really
necessary? If you did,
* s/bio_split()/bio_pair_split().
* introduce better and prettier bio_split() which has
different semantics.
* replace bio_pair_split() users with bio_split().
the renaming would have made sense, but you renamed an existing API,
intrudced a new one and then changed the renamed old API. Doesn't
make too much sense to me.
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 0f31ec4..9fa07c7 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1080,15 +1080,9 @@ static void make_request(struct mddev *mddev, struct bio * bio)
> && (conf->geo.near_copies < conf->geo.raid_disks
> || 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)
> - goto bad_map;
> - /* This is a one page bio that upper layers
> - * refuse to split for us, so we need to split it.
> - */
> +
> bp = bio_pair_split(bio,
> - chunk_sects - (bio->bi_sector & (chunk_sects - 1)) );
> + chunk_sects - (bio->bi_sector & (chunk_sects - 1)));
I suppose this one too belongs to the previous rename patch?
> --- a/fs/bio-integrity.c
> +++ b/fs/bio-integrity.c
> @@ -681,50 +681,6 @@ void bio_integrity_trim(struct bio *bio, unsigned int offset,
> EXPORT_SYMBOL(bio_integrity_trim);
>
> /**
> - * bio_integrity_split - Split integrity metadata
> - * @bio: Protected bio
> - * @bp: Resulting bio_pair
> - * @sectors: Offset
> - *
> - * Description: Splits an integrity page into a bio_pair.
> - */
> -void bio_integrity_split(struct bio *bio, struct bio_pair *bp, int sectors)
> -{
> - struct blk_integrity *bi;
> - struct bio_integrity_payload *bip = bio->bi_integrity;
> - unsigned int nr_sectors;
> -
> - if (bio_integrity(bio) == 0)
> - return;
> -
> - bi = bdev_get_integrity(bio->bi_bdev);
> - BUG_ON(bi == NULL);
> - BUG_ON(bip->bip_vcnt != 1);
> -
> - nr_sectors = bio_integrity_hw_sectors(bi, sectors);
> -
> - bp->bio1.bi_integrity = &bp->bip1;
> - bp->bio2.bi_integrity = &bp->bip2;
> -
> - bp->iv1 = bip->bip_vec[0];
> - bp->iv2 = bip->bip_vec[0];
> -
> - bp->bip1.bip_vec[0] = bp->iv1;
> - bp->bip2.bip_vec[0] = bp->iv2;
> -
> - bp->iv1.bv_len = sectors * bi->tuple_size;
> - bp->iv2.bv_offset += sectors * bi->tuple_size;
> - bp->iv2.bv_len -= sectors * bi->tuple_size;
> -
> - bp->bip1.bip_sector = bio->bi_integrity->bip_sector;
> - bp->bip2.bip_sector = bio->bi_integrity->bip_sector + nr_sectors;
> -
> - bp->bip1.bip_vcnt = bp->bip2.bip_vcnt = 1;
> - bp->bip1.bip_idx = bp->bip2.bip_idx = 0;
> -}
> -EXPORT_SYMBOL(bio_integrity_split);
I complained about this in the last posting and in the previous patch.
Please respond. Martin, are you okay with these integrity changes?
> -static void bio_pair_end_1(struct bio *bi, int err)
> +static void bio_pair_end(struct bio *bio, int error)
> {
> - struct bio_pair *bp = container_of(bi, struct bio_pair, bio1);
> + struct bio_pair *bp = bio->bi_private;
>
> - if (err)
> - bp->error = err;
> -
> - bio_pair_release(bp);
> -}
> -
> -static void bio_pair_end_2(struct bio *bi, int err)
> -{
> - struct bio_pair *bp = container_of(bi, struct bio_pair, bio2);
> -
> - if (err)
> - bp->error = err;
> + if (error)
> + clear_bit(BIO_UPTODATE, &bp->orig->bi_flags);
>
> bio_pair_release(bp);
> }
Why is losing error value okay here?
> @@ -1856,8 +1846,7 @@ static int __init init_bio(void)
> if (bioset_integrity_create(fs_bio_set, BIO_POOL_SIZE))
> panic("bio: can't create integrity pool\n");
>
> - bio_split_pool = mempool_create_kmalloc_pool(BIO_SPLIT_ENTRIES,
> - sizeof(struct bio_pair));
> + bio_split_pool = bioset_create(BIO_POOL_SIZE, offsetof(struct bio_pair, split));
> if (!bio_split_pool)
> panic("bio: can't create split pool\n");
It would be nice to mention that using this from stacking drivers is
inherently broken. This is something which has been broken before
this patch but still. bio_split*() should always require separate
biosets.
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 1c3bb47..3ad3540 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -192,14 +192,13 @@ struct bio_integrity_payload {
> * in bio2.bi_private
> */
> struct bio_pair {
> - struct bio bio1, bio2;
> - struct bio_vec bv1, bv2;
> -#if defined(CONFIG_BLK_DEV_INTEGRITY)
> - struct bio_integrity_payload bip1, bip2;
> - struct bio_vec iv1, iv2;
> -#endif
> - atomic_t cnt;
> - int error;
> + atomic_t cnt;
> +
> + bio_end_io_t *bi_end_io;
> + void *bi_private;
> +
> + struct bio *orig;
> + struct bio split;
> };
So, this is struct is allocated as frontpad, which is a pretty unusual
thing to do. Please explain and emphasize that ->split should come
last. Also, given that it's a pair split, it would be nice to somehow
indicate that ->split is the earlier half. Before this change it was
pretty clear with ->bio1/2.
Thanks.
--
tejun
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