md/raid10: fix two problems with RAID10 resync.

1/ When an different between blocks is found, data is copied from
   one bio to the other.  However bv_len is used as the length to
   copy and this could be zero.  So use r10_bio->sectors to calculate
   length instead.
   Using bv_len was probably always a bit dubious, but the introduction
   of bio_advance made it much more likely to be a problem.

2/ When preparing some blocks for sync, we don't set BIO_UPTODATE
   except on bios that we schedule for a read.  This ensures that
   missing/failed devices don't confuse the loop at the top of
   sync_request write.
   Commit 8be185f2c9d54d6 "raid10: Use bio_reset()"
   removed a loop which set BIO_UPTDATE on all appropriate bios.
   So we need to re-add that flag.

These bugs were introduced in 3.10, so this patch is suitable for
3.10-stable, and can remove a potential for data corruption.

Cc: stable@vger.kernel.org (3.10)
Reported-by: Brassow Jonathan <jbrassow@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index cd066b63bdaf..957a719e8c2f 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -2097,11 +2097,17 @@ static void sync_request_write(struct mddev *mddev, struct r10bio *r10_bio)
                          * both 'first' and 'i', so we just compare them.
                          * All vec entries are PAGE_SIZE;
                          */
-                        for (j = 0; j < vcnt; j++)
+                        int sectors = r10_bio->sectors;
+                        for (j = 0; j < vcnt; j++) {
+                                int len = PAGE_SIZE;
+                                if (sectors < (len / 512))
+                                        len = sectors * 512;
                                 if (memcmp(page_address(fbio->bi_io_vec[j].bv_page),
                                            page_address(tbio->bi_io_vec[j].bv_page),
-                                           fbio->bi_io_vec[j].bv_len))
+                                           len))
                                         break;
+                                sectors -= len/512;
+                        }
                         if (j == vcnt)
                                 continue;
                         atomic64_add(r10_bio->sectors, &mddev->resync_mismatches);
@@ -3407,6 +3413,7 @@ static sector_t sync_request(struct mddev *mddev, sector_t sector_nr,
 
                 if (bio->bi_end_io == end_sync_read) {
                         md_sync_acct(bio->bi_bdev, nr_sectors);
+                        set_bit(BIO_UPTODATE, &bio->bi_flags);
                         generic_make_request(bio);
                 }
         }