From: Jens Axboe <axboe@suse.de>
Although it was an established part of the current bio api, it was never
documented that bio_add_page() and merge_bvec_fn() must accept to add at
least one page to an empty bio.
---
drivers/block/ll_rw_blk.c | 8 +++++---
fs/bio.c | 4 +++-
2 files changed, 8 insertions(+), 4 deletions(-)
diff -puN drivers/block/ll_rw_blk.c~bio-documentation-update drivers/block/ll_rw_blk.c
--- 25/drivers/block/ll_rw_blk.c~bio-documentation-update 2004-01-09 00:47:29.000000000 -0800
+++ 25-akpm/drivers/block/ll_rw_blk.c 2004-01-09 00:47:29.000000000 -0800
@@ -174,9 +174,11 @@ EXPORT_SYMBOL(blk_queue_prep_rq);
* are dynamic, and thus we have to query the queue whether it is ok to
* add a new bio_vec to a bio at a given offset or not. If the block device
* has such limitations, it needs to register a merge_bvec_fn to control
- * the size of bio's sent to it. Per default now merge_bvec_fn is defined for
- * a queue, and only the fixed limits are honored.
- *
+ * the size of bio's sent to it. Note that a block device *must* allow a
+ * single page to be added to an empty bio. The block device driver may want
+ * to use the bio_split() function to deal with these bio's. By default
+ * no merge_bvec_fn is defined for a queue, and only the fixed limits are
+ * honored.
*/
void blk_queue_merge_bvec(request_queue_t *q, merge_bvec_fn *mbfn)
{
diff -puN fs/bio.c~bio-documentation-update fs/bio.c
--- 25/fs/bio.c~bio-documentation-update 2004-01-09 00:47:29.000000000 -0800
+++ 25-akpm/fs/bio.c 2004-01-09 00:47:29.000000000 -0800
@@ -290,7 +290,9 @@ int bio_get_nr_vecs(struct block_device
*
* Attempt to add a page to the bio_vec maplist. This can fail for a
* number of reasons, such as the bio being full or target block
- * device limitations.
+ * device limitations. The target block device must allow bio's
+ * smaller than PAGE_SIZE, so it is always possible to add a single
+ * page to an empty bio.
*/
int bio_add_page(struct bio *bio, struct page *page, unsigned int len,
unsigned int offset)
_