bcache: use bvec_kmap_local in bch_data_verify

Using local kmaps slightly reduces the chances to stray writes, and
the bvec interface cleans up the code a little bit.

Also switch from page_address to bvec_kmap_local for cbv to be on the
safe side and to avoid pointlessly poking into bvec internals.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20211020143812.6403-8-colyli@suse.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/debug.c

@@ -127,21 +127,20 @@ void bch_data_verify(struct cached_dev *dc, struct bio *bio)
 
 	citer.bi_size = UINT_MAX;
 	bio_for_each_segment(bv, bio, iter) {
-		void *p1 = kmap_atomic(bv.bv_page);
+		void *p1 = bvec_kmap_local(&bv);
 		void *p2;
 
 		cbv = bio_iter_iovec(check, citer);
-		p2 = page_address(cbv.bv_page);
+		p2 = bvec_kmap_local(&cbv);
 
-		cache_set_err_on(memcmp(p1 + bv.bv_offset,
-					p2 + bv.bv_offset,
-					bv.bv_len),
+		cache_set_err_on(memcmp(p1, p2, bv.bv_len),
 				 dc->disk.c,
 				 "verify failed at dev %pg sector %llu",
 				 dc->bdev,
 				 (uint64_t) bio->bi_iter.bi_sector);
 
-		kunmap_atomic(p1);
+		kunmap_local(p2);
+		kunmap_local(p1);
 		bio_advance_iter(check, &citer, bv.bv_len);
 	}