dm: cleanup of front padding calculation

Add two helper macros calculating the offset of bio in struct dm_io and
struct dm_target_io respectively.

Besides, simplify the front padding calculation in
dm_alloc_md_mempools().
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -105,12 +105,16 @@ struct dm_io {
 	struct dm_target_io tio;
 };
 
+#define DM_TARGET_IO_BIO_OFFSET (offsetof(struct dm_target_io, clone))
+#define DM_IO_BIO_OFFSET \
+	(offsetof(struct dm_target_io, clone) + offsetof(struct dm_io, tio))
+
 void *dm_per_bio_data(struct bio *bio, size_t data_size)
 {
 	struct dm_target_io *tio = container_of(bio, struct dm_target_io, clone);
 	if (!tio->inside_dm_io)
-		return (char *)bio - offsetof(struct dm_target_io, clone) - data_size;
-	return (char *)bio - offsetof(struct dm_target_io, clone) - offsetof(struct dm_io, tio) - data_size;
+		return (char *)bio - DM_TARGET_IO_BIO_OFFSET - data_size;
+	return (char *)bio - DM_IO_BIO_OFFSET - data_size;
 }
 EXPORT_SYMBOL_GPL(dm_per_bio_data);
 
@@ -118,9 +122,9 @@ struct bio *dm_bio_from_per_bio_data(void *data, size_t data_size)
 {
 	struct dm_io *io = (struct dm_io *)((char *)data + data_size);
 	if (io->magic == DM_IO_MAGIC)
-		return (struct bio *)((char *)io + offsetof(struct dm_io, tio) + offsetof(struct dm_target_io, clone));
+		return (struct bio *)((char *)io + DM_IO_BIO_OFFSET);
 	BUG_ON(io->magic != DM_TIO_MAGIC);
-	return (struct bio *)((char *)io + offsetof(struct dm_target_io, clone));
+	return (struct bio *)((char *)io + DM_TARGET_IO_BIO_OFFSET);
 }
 EXPORT_SYMBOL_GPL(dm_bio_from_per_bio_data);
 
@@ -2849,8 +2853,8 @@ struct dm_md_mempools *dm_alloc_md_mempools(struct mapped_device *md, enum dm_qu
 	case DM_TYPE_BIO_BASED:
 	case DM_TYPE_DAX_BIO_BASED:
 		pool_size = max(dm_get_reserved_bio_based_ios(), min_pool_size);
-		front_pad = roundup(per_io_data_size, __alignof__(struct dm_target_io)) + offsetof(struct dm_target_io, clone);
-		io_front_pad = roundup(front_pad,  __alignof__(struct dm_io)) + offsetof(struct dm_io, tio);
+		front_pad = roundup(per_io_data_size, __alignof__(struct dm_target_io)) + DM_TARGET_IO_BIO_OFFSET;
+		io_front_pad = roundup(per_io_data_size,  __alignof__(struct dm_io)) + DM_IO_BIO_OFFSET;
 		ret = bioset_init(&pools->io_bs, pool_size, io_front_pad, 0);
 		if (ret)
 			goto out;