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Date: Wed, 25 Jul 2012 14:55:40 +0300
From: Boaz Harrosh <bharrosh@...asas.com>
To: Kent Overstreet <koverstreet@...gle.com>
CC: <linux-bcache@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<dm-devel@...hat.com>, <tj@...nel.org>, <axboe@...nel.dk>,
<agk@...hat.com>, <neilb@...e.de>, <drbd-dev@...ts.linbit.com>,
<vgoyal@...hat.com>, <mpatocka@...hat.com>, <sage@...dream.net>,
<yehuda@...newdream.net>
Subject: Re: [PATCH v4 08/12] block: Introduce new bio_split()
On 07/24/2012 11:11 PM, Kent Overstreet wrote:
> The new bio_split() can split arbitrary bios - it's not restricted to
> single page bios, like the old bio_split() (previously renamed to
> bio_pair_split()). It also has different semantics - it doesn't allocate
> a struct bio_pair, leaving it up to the caller to handle completions.
>
> Signed-off-by: Kent Overstreet <koverstreet@...gle.com>
> ---
> fs/bio.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 99 insertions(+), 0 deletions(-)
>
> diff --git a/fs/bio.c b/fs/bio.c
> index 5d02aa5..a15e121 100644
> --- a/fs/bio.c
> +++ b/fs/bio.c
> @@ -1539,6 +1539,105 @@ struct bio_pair *bio_pair_split(struct bio *bi, int first_sectors)
> EXPORT_SYMBOL(bio_pair_split);
>
> /**
> + * bio_split - split a bio
> + * @bio: bio to split
> + * @sectors: number of sectors to split from the front of @bio
> + * @gfp: gfp mask
> + * @bs: bio set to allocate from
> + *
> + * Allocates and returns a new bio which represents @sectors from the start of
> + * @bio, and updates @bio to represent the remaining sectors.
> + *
> + * If bio_sectors(@bio) was less than or equal to @sectors, returns @bio
> + * unchanged.
> + *
> + * The newly allocated bio will point to @bio's bi_io_vec, if the split was on a
> + * bvec boundry; it is the caller's responsibility to ensure that @bio is not
> + * freed before the split.
> + *
> + * If bio_split() is running under generic_make_request(), it's not safe to
> + * allocate more than one bio from the same bio set. Therefore, if it is running
> + * under generic_make_request() it masks out __GFP_WAIT when doing the
> + * allocation. The caller must check for failure if there's any possibility of
> + * it being called from under generic_make_request(); it is then the caller's
> + * responsibility to retry from a safe context (by e.g. punting to workqueue).
> + */
> +struct bio *bio_split(struct bio *bio, int sectors,
> + gfp_t gfp, struct bio_set *bs)
> +{
> + unsigned idx, vcnt = 0, nbytes = sectors << 9;
> + struct bio_vec *bv;
> + struct bio *ret = NULL;
> +
> + BUG_ON(sectors <= 0);
> +
> + /*
> + * If we're being called from underneath generic_make_request() and we
> + * already allocated any bios from this bio set, we risk deadlock if we
> + * use the mempool. So instead, we possibly fail and let the caller punt
> + * to workqueue or somesuch and retry in a safe context.
> + */
> + if (current->bio_list)
> + gfp &= ~__GFP_WAIT;
NACK!
If as you said above in the comment:
if there's any possibility of it being called from under generic_make_request();
it is then the caller's responsibility to ...
So all the comment needs to say is:
... caller's responsibility to not set __GFP_WAIT at gfp.
And drop this here. It is up to the caller to decide. If the caller wants he can do
"if (current->bio_list)" by his own.
This is a general purpose utility you might not know it's context.
for example with osdblk above will break.
Thanks
Boaz
> +
> + if (sectors >= bio_sectors(bio))
> + return bio;
> +
> + trace_block_split(bdev_get_queue(bio->bi_bdev), bio,
> + bio->bi_sector + sectors);
> +
> + bio_for_each_segment(bv, bio, idx) {
> + vcnt = idx - bio->bi_idx;
> +
> + if (!nbytes) {
> + ret = bio_alloc_bioset(gfp, 0, bs);
> + if (!ret)
> + return NULL;
> +
> + ret->bi_io_vec = bio_iovec(bio);
> + ret->bi_flags |= 1 << BIO_CLONED;
> + break;
> + } else if (nbytes < bv->bv_len) {
> + ret = bio_alloc_bioset(gfp, ++vcnt, bs);
> + if (!ret)
> + return NULL;
> +
> + memcpy(ret->bi_io_vec, bio_iovec(bio),
> + sizeof(struct bio_vec) * vcnt);
> +
> + ret->bi_io_vec[vcnt - 1].bv_len = nbytes;
> + bv->bv_offset += nbytes;
> + bv->bv_len -= nbytes;
> + break;
> + }
> +
> + nbytes -= bv->bv_len;
> + }
> +
> + ret->bi_bdev = bio->bi_bdev;
> + ret->bi_sector = bio->bi_sector;
> + ret->bi_size = sectors << 9;
> + ret->bi_rw = bio->bi_rw;
> + ret->bi_vcnt = vcnt;
> + ret->bi_max_vecs = vcnt;
> + ret->bi_end_io = bio->bi_end_io;
> + ret->bi_private = bio->bi_private;
> +
> + bio->bi_sector += sectors;
> + bio->bi_size -= sectors << 9;
> + bio->bi_idx = idx;
> +
> + if (bio_integrity(bio)) {
> + bio_integrity_clone(ret, bio, gfp, bs);
> + bio_integrity_trim(ret, 0, bio_sectors(ret));
> + bio_integrity_trim(bio, bio_sectors(ret), bio_sectors(bio));
> + }
> +
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(bio_split);
> +
> +/**
> * bio_sector_offset - Find hardware sector offset in bio
> * @bio: bio to inspect
> * @index: bio_vec index
--
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