dm io: fix discard support

This patch fixes a crash by recognising discards in dm_io.

Currently dm_mirror can send REQ_DISCARD bios if running over a
discard-enabled device and without support in dm_io the system
crashes badly.

BUG: unable to handle kernel paging request at 00800000
IP:  __bio_add_page.part.17+0xf5/0x1e0
...
 bio_add_page+0x56/0x70
 dispatch_io+0x1cf/0x240 [dm_mod]
 ? km_get_page+0x50/0x50 [dm_mod]
 ? vm_next_page+0x20/0x20 [dm_mod]
 ? mirror_flush+0x130/0x130 [dm_mirror]
 dm_io+0xdc/0x2b0 [dm_mod]
...

Introduced in 2.6.38-rc1 by commit 5fc2ffeabb9ee0fc0e71ff16b49f34f0ed3d05b4
(dm raid1: support discard).

Signed-off-by: Milan Broz <mbroz@redhat.com>
Cc: stable@kernel.org
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 16 insertions, 7 deletions

diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
index ad2eba40e319..ea5dd289fe2a 100644
--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -296,6 +296,8 @@ static void do_region(int rw, unsigned region, struct dm_io_region *where,
         unsigned offset;
         unsigned num_bvecs;
         sector_t remaining = where->count;
+        struct request_queue *q = bdev_get_queue(where->bdev);
+        sector_t discard_sectors;
 
         /*
          * where->count may be zero if rw holds a flush and we need to
@@ -305,9 +307,12 @@ static void do_region(int rw, unsigned region, struct dm_io_region *where,
                 /*
                  * Allocate a suitably sized-bio.
                  */
-                num_bvecs = dm_sector_div_up(remaining,
-                                             (PAGE_SIZE >> SECTOR_SHIFT));
-                num_bvecs = min_t(int, bio_get_nr_vecs(where->bdev), num_bvecs);
+                if (rw & REQ_DISCARD)
+                        num_bvecs = 1;
+                else
+                        num_bvecs = min_t(int, bio_get_nr_vecs(where->bdev),
+                                          dm_sector_div_up(remaining, (PAGE_SIZE >> SECTOR_SHIFT)));
+
                 bio = bio_alloc_bioset(GFP_NOIO, num_bvecs, io->client->bios);
                 bio->bi_sector = where->sector + (where->count - remaining);
                 bio->bi_bdev = where->bdev;
@@ -315,10 +320,14 @@ static void do_region(int rw, unsigned region, struct dm_io_region *where,
                 bio->bi_destructor = dm_bio_destructor;
                 store_io_and_region_in_bio(bio, io, region);
 
-                /*
-                 * Try and add as many pages as possible.
-                 */
-                while (remaining) {
+                if (rw & REQ_DISCARD) {
+                        discard_sectors = min_t(sector_t, q->limits.max_discard_sectors, remaining);
+                        bio->bi_size = discard_sectors << SECTOR_SHIFT;
+                        remaining -= discard_sectors;
+                } else while (remaining) {
+                        /*
+                         * Try and add as many pages as possible.
+                         */
                         dp->get_page(dp, &page, &len, &offset);
                         len = min(len, to_bytes(remaining));
                         if (!bio_add_page(bio, page, len, offset))