[PATCH v2 07/51] bcache: comment on direct access to bvec table

Ming Lei
2017-06-26T12:13:56Z

Looks all are safe after multipage bvec is supported.

Cc: linux-bcache@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.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 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;
--
2.9.4