1 files changed, 35 insertions, 16 deletions

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 4858d925b95e..76fbfa120064 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -1566,22 +1566,41 @@ static int rbd_merge_bvec(struct request_queue *q, struct bvec_merge_data *bmd,
                           struct bio_vec *bvec)
 {
         struct rbd_device *rbd_dev = q->queuedata;
-        unsigned int chunk_sectors;
-        sector_t sector;
-        unsigned int bio_sectors;
-        int max;
-
-        chunk_sectors = 1 << (rbd_dev->header.obj_order - SECTOR_SHIFT);
-        sector = bmd->bi_sector + get_start_sect(bmd->bi_bdev);
-        bio_sectors = bmd->bi_size >> SECTOR_SHIFT;
-
-        max =  (chunk_sectors - ((sector & (chunk_sectors - 1))
-                                 + bio_sectors)) << SECTOR_SHIFT;
-        if (max < 0)
-                max = 0; /* bio_add cannot handle a negative return */
-        if (max <= bvec->bv_len && bio_sectors == 0)
-                return bvec->bv_len;
-        return max;
+        sector_t sector_offset;
+        sector_t sectors_per_obj;
+        sector_t obj_sector_offset;
+        int ret;
+
+        /*
+         * Find how far into its rbd object the partition-relative
+         * bio start sector is to offset relative to the enclosing
+         * device.
+         */
+        sector_offset = get_start_sect(bmd->bi_bdev) + bmd->bi_sector;
+        sectors_per_obj = 1 << (rbd_dev->header.obj_order - SECTOR_SHIFT);
+        obj_sector_offset = sector_offset & (sectors_per_obj - 1);
+
+        /*
+         * Compute the number of bytes from that offset to the end
+         * of the object.  Account for what's already used by the bio.
+         */
+        ret = (int) (sectors_per_obj - obj_sector_offset) << SECTOR_SHIFT;
+        if (ret > bmd->bi_size)
+                ret -= bmd->bi_size;
+        else
+                ret = 0;
+
+        /*
+         * Don't send back more than was asked for.  And if the bio
+         * was empty, let the whole thing through because:  "Note
+         * that a block device *must* allow a single page to be
+         * added to an empty bio."
+         */
+        rbd_assert(bvec->bv_len <= PAGE_SIZE);
+        if (ret > (int) bvec->bv_len || !bmd->bi_size)
+                ret = (int) bvec->bv_len;
+
+        return ret;
 }
 
 static void rbd_free_disk(struct rbd_device *rbd_dev)