diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index fce4cbf9529d..50f3e673729c 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -1526,6 +1526,10 @@ EXPORT_SYMBOL_GPL(dm_bufio_get_block_size);
 sector_t dm_bufio_get_device_size(struct dm_bufio_client *c)
 {
 	sector_t s = i_size_read(c->bdev->bd_inode) >> SECTOR_SHIFT;
+	if (s >= c->start)
+		s -= c->start;
+	else
+		s = 0;
 	if (likely(c->sectors_per_block_bits >= 0))
 		s >>= c->sectors_per_block_bits;
 	else
diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index fb41b4f23c48..66f4c6398f67 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -61,19 +61,18 @@ static int fec_decode_rs8(struct dm_verity *v, struct dm_verity_fec_io *fio,
 static u8 *fec_read_parity(struct dm_verity *v, u64 rsb, int index,
 			   unsigned *offset, struct dm_buffer **buf)
 {
-	u64 position, block;
+	u64 position, block, rem;
 	u8 *res;
 
 	position = (index + rsb) * v->fec->roots;
-	block = position >> v->data_dev_block_bits;
-	*offset = (unsigned)(position - (block << v->data_dev_block_bits));
+	block = div64_u64_rem(position, v->fec->roots << SECTOR_SHIFT, &rem);
+	*offset = (unsigned)rem;
 
-	res = dm_bufio_read(v->fec->bufio, v->fec->start + block, buf);
+	res = dm_bufio_read(v->fec->bufio, block, buf);
 	if (IS_ERR(res)) {
 		DMERR("%s: FEC %llu: parity read failed (block %llu): %ld",
 		      v->data_dev->name, (unsigned long long)rsb,
-		      (unsigned long long)(v->fec->start + block),
-		      PTR_ERR(res));
+		      (unsigned long long)block, PTR_ERR(res));
 		*buf = NULL;
 	}
 
@@ -155,7 +154,7 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_fec_io *fio,
 
 		/* read the next block when we run out of parity bytes */
 		offset += v->fec->roots;
-		if (offset >= 1 << v->data_dev_block_bits) {
+		if (offset >= v->fec->roots << SECTOR_SHIFT) {
 			dm_bufio_release(buf);
 
 			par = fec_read_parity(v, rsb, block_offset, &offset, &buf);
@@ -674,7 +673,7 @@ int verity_fec_ctr(struct dm_verity *v)
 {
 	struct dm_verity_fec *f = v->fec;
 	struct dm_target *ti = v->ti;
-	u64 hash_blocks;
+	u64 hash_blocks, fec_blocks;
 	int ret;
 
 	if (!verity_fec_is_enabled(v)) {
@@ -744,15 +743,17 @@ int verity_fec_ctr(struct dm_verity *v)
 	}
 
 	f->bufio = dm_bufio_client_create(f->dev->bdev,
-					  1 << v->data_dev_block_bits,
+					  f->roots << SECTOR_SHIFT,
 					  1, 0, NULL, NULL);
 	if (IS_ERR(f->bufio)) {
 		ti->error = "Cannot initialize FEC bufio client";
 		return PTR_ERR(f->bufio);
 	}
 
-	if (dm_bufio_get_device_size(f->bufio) <
-	    ((f->start + f->rounds * f->roots) >> v->data_dev_block_bits)) {
+	dm_bufio_set_sector_offset(f->bufio, f->start << (v->data_dev_block_bits - SECTOR_SHIFT));
+
+	fec_blocks = div64_u64(f->rounds * f->roots, v->fec->roots << SECTOR_SHIFT);
+	if (dm_bufio_get_device_size(f->bufio) < fec_blocks) {
 		ti->error = "FEC device is too small";
 		return -E2BIG;
 	}