md: raid1: Fix restoration of bio between failed read and write.

When performing a "recovery" or "check" pass on a RAID1 array, we read
from each device and possible, if there is a difference or a read error,
write back to some devices.

We use the same 'bio' for both read and write, resetting various fields
between the two operations.

We forgot to reset bv_offset and bv_len however.  These are often left
unchanged, but in the case where there is an IO error one or two sectors
into a page, they are changed.

This results in correctable errors not being corrected properly.  It does
not result in any data corruption.

Cc: "Fairbanks, David" <David.Fairbanks@stratus.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/raid1.c

@@ -1284,6 +1284,7 @@ static void sync_request_write(mddev_t *mddev, r1bio_t *r1_bio)
 					rdev_dec_pending(conf->mirrors[i].rdev, mddev);
 				} else {
 					/* fixup the bio for reuse */
+					int size;
 					sbio->bi_vcnt = vcnt;
 					sbio->bi_size = r1_bio->sectors << 9;
 					sbio->bi_idx = 0;
@@ -1297,10 +1298,20 @@ static void sync_request_write(mddev_t *mddev, r1bio_t *r1_bio)
 					sbio->bi_sector = r1_bio->sector +
 						conf->mirrors[i].rdev->data_offset;
 					sbio->bi_bdev = conf->mirrors[i].rdev->bdev;
-					for (j = 0; j < vcnt ; j++)
-						memcpy(page_address(sbio->bi_io_vec[j].bv_page),
+					size = sbio->bi_size;
+					for (j = 0; j < vcnt ; j++) {
+						struct bio_vec *bi;
+						bi = &sbio->bi_io_vec[j];
+						bi->bv_offset = 0;
+						if (size > PAGE_SIZE)
+							bi->bv_len = PAGE_SIZE;
+						else
+							bi->bv_len = size;
+						size -= PAGE_SIZE;
+						memcpy(page_address(bi->bv_page),
 						       page_address(pbio->bi_io_vec[j].bv_page),
 						       PAGE_SIZE);
+					}
 
 				}
 			}