linux-kernel - Re: [PATCH v3 07/49] bcache: comment on direct access to bvec table

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Date:   Tue, 8 Aug 2017 20:36:21 +0800
From:   Coly Li <i@...y.li>
To:     Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@...radead.org>,
        Huang Ying <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-bcache@...r.kernel.org
Subject: Re: [PATCH v3 07/49] bcache: comment on direct access to bvec table

On 2017/8/8 下午4:45, Ming Lei wrote:
> Looks all are safe after multipage bvec is supported.
> 
> Cc: linux-bcache@...r.kernel.org
> Signed-off-by: Ming Lei <ming.lei@...hat.com>

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

Coly Li


> ---
>  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 866dcf78ff8e..3da595ae565b 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -431,6 +431,7 @@ 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 8352fad765f6..6808f548cd13 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;
>  
> @@ -1154,6 +1155,8 @@ 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 8c3a938f4bf0..11b4230ea6ad 100644
> --- a/drivers/md/bcache/util.c
> +++ b/drivers/md/bcache/util.c
> @@ -223,6 +223,13 @@ 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;
>