Re: [PATCH v1 07/54] bcache: comment on direct access to bvec table

From: Coly Li <colyli@suse.de>
To: Ming Lei <tom.leiming@gmail.com>
Cc: Jens Axboe <axboe@fb.com>,
	linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	Kent Overstreet <kent.overstreet@gmail.com>,
	Shaohua Li <shli@kernel.org>, Guoqing Jiang <gqjiang@suse.com>,
	Zheng Liu <gnehzuil.liu@gmail.com>,
	Mike Christie <mchristi@redhat.com>,
	Jiri Kosina <jkosina@suse.cz>,
	Eric Wheeler <git@linux.ewheeler.net>,
	Yijing Wang <wangyijing@huawei.com>,
	Al Viro <viro@ZenIV.linux.org.uk>,
	"open list:BCACHE (BLOCK LAYER CACHE)"
	<linux-bcache@vger.kernel.org>,
	"open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
	<linux-raid@vger.kernel.org>
Subject: Re: [PATCH v1 07/54] bcache: comment on direct access to bvec table
Date: Sat, 31 Dec 2016 00:56:41 +0800	[thread overview]
Message-ID: <f88ea6e0-d2e4-15ea-0eb6-19be3d9d317d@suse.de> (raw)
In-Reply-To: <1482854250-13481-8-git-send-email-tom.leiming@gmail.com>

On 2016/12/27 下午11:55, Ming Lei wrote:
> Looks all are safe after multipage bvec is supported.
> 
> Signed-off-by: Ming Lei <tom.leiming@gmail.com>
> ---
>  drivers/md/bcache/btree.c | 1 +
>  drivers/md/bcache/super.c | 6 ++++++
>  drivers/md/bcache/util.c  | 7 +++++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index a43eedd5804d..fc35cfb4d0f1 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -428,6 +428,7 @@ static void do_btree_node_write(struct btree *b)
>  
>  		continue_at(cl, btree_node_write_done, NULL);
>  	} else {
> +		/* No harm for multipage bvec since the new is just allocated */
>  		b->bio->bi_vcnt = 0;
>  		bch_bio_map(b->bio, i);
>  
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 3a19cbc8b230..607b022259dc 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -208,6 +208,7 @@ static void write_bdev_super_endio(struct bio *bio)
>  
>  static void __write_super(struct cache_sb *sb, struct bio *bio)
>  {
> +	/* single page bio, safe for multipage bvec */
>  	struct cache_sb *out = page_address(bio->bi_io_vec[0].bv_page);
>  	unsigned i;
>  
> @@ -1156,6 +1157,8 @@ static void register_bdev(struct cache_sb *sb, struct page *sb_page,
>  	dc->bdev->bd_holder = dc;
>  
>  	bio_init(&dc->sb_bio, dc->sb_bio.bi_inline_vecs, 1);
> +
> +	/* single page bio, safe for multipage bvec */
>  	dc->sb_bio.bi_io_vec[0].bv_page = sb_page;
>  	get_page(sb_page);
>  
> @@ -1799,6 +1802,7 @@ void bch_cache_release(struct kobject *kobj)
>  	for (i = 0; i < RESERVE_NR; i++)
>  		free_fifo(&ca->free[i]);
>  
> +	/* single page bio, safe for multipage bvec */
>  	if (ca->sb_bio.bi_inline_vecs[0].bv_page)
>  		put_page(ca->sb_bio.bi_io_vec[0].bv_page);
>  
> @@ -1854,6 +1858,8 @@ static int register_cache(struct cache_sb *sb, struct page *sb_page,
>  	ca->bdev->bd_holder = ca;
>  
>  	bio_init(&ca->sb_bio, ca->sb_bio.bi_inline_vecs, 1);
> +
> +	/* single page bio, safe for multipage bvec */
>  	ca->sb_bio.bi_io_vec[0].bv_page = sb_page;
>  	get_page(sb_page);
>  
> diff --git a/drivers/md/bcache/util.c b/drivers/md/bcache/util.c
> index dde6172f3f10..5cc0b49a65fb 100644
> --- a/drivers/md/bcache/util.c
> +++ b/drivers/md/bcache/util.c
> @@ -222,6 +222,13 @@ uint64_t bch_next_delay(struct bch_ratelimit *d, uint64_t done)
>  		: 0;
>  }
>  
> +/*
> + * Generally it isn't good to access .bi_io_vec and .bi_vcnt
> + * directly, the preferred way is bio_add_page, but in
> + * this case, bch_bio_map() supposes that the bvec table
> + * is empty, so it is safe to access .bi_vcnt & .bi_io_vec
> + * in this way even after multipage bvec is supported.
> + */
>  void bch_bio_map(struct bio *bio, void *base)
>  {
>  	size_t size = bio->bi_iter.bi_size;
> 

Acked-by: Coly Li <colyli@suse.de>