Add bvec_merge_data to handle stacked devices and ->merge_bvec()

When devices are stacked, one device's merge_bvec_fn may need to perform
the mapping and then call one or more functions for its underlying devices.

The following bio fields are used:
  bio->bi_sector
  bio->bi_bdev
  bio->bi_size
  bio->bi_rw  using bio_data_dir()

This patch creates a new struct bvec_merge_data holding a copy of those
fields to avoid having to change them directly in the struct bio when
going down the stack only to have to change them back again on the way
back up.  (And then when the bio gets mapped for real, the whole
exercise gets repeated, but that's a problem for another day...)

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Milan Broz <mbroz@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/drivers/md/linear.c b/drivers/md/linear.c
index 10748240cb2f..6a866d7c8ae5 100644
--- a/drivers/md/linear.c
+++ b/drivers/md/linear.c
@@ -50,17 +50,19 @@ static inline dev_info_t *which_dev(mddev_t *mddev, sector_t sector)
 /**
  *      linear_mergeable_bvec -- tell bio layer if two requests can be merged
  *      @q: request queue
- *      @bio: the buffer head that's been built up so far
+ *      @bvm: properties of new bio
  *      @biovec: the request that could be merged to it.
  *
  *      Return amount of bytes we can take at this offset
  */
-static int linear_mergeable_bvec(struct request_queue *q, struct bio *bio, struct bio_vec *biovec)
+static int linear_mergeable_bvec(struct request_queue *q,
+                                 struct bvec_merge_data *bvm,
+                                 struct bio_vec *biovec)
 {
         mddev_t *mddev = q->queuedata;
         dev_info_t *dev0;
-        unsigned long maxsectors, bio_sectors = bio->bi_size >> 9;
-        sector_t sector = bio->bi_sector + get_start_sect(bio->bi_bdev);
+        unsigned long maxsectors, bio_sectors = bvm->bi_size >> 9;
+        sector_t sector = bvm->bi_sector + get_start_sect(bvm->bi_bdev);
 
         dev0 = which_dev(mddev, sector);
         maxsectors = (dev0->size << 1) - (sector - (dev0->offset<<1));