btrfs: factor out a btrfs_csum_ptr helper

Add a helper to find the csum for a byte offset into the csum buffer.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -2733,6 +2733,14 @@ int btrfs_get_extent_inline_ref_type(const struct extent_buffer *eb,
 				     enum btrfs_inline_ref_type is_data);
 u64 hash_extent_data_ref(u64 root_objectid, u64 owner, u64 offset);
 
+static inline u8 *btrfs_csum_ptr(const struct btrfs_fs_info *fs_info, u8 *csums,
+				 u64 offset)
+{
+	u64 offset_in_sectors = offset >> fs_info->sectorsize_bits;
+
+	return csums + offset_in_sectors * fs_info->csum_size;
+}
+
 /*
  * Take the number of bytes to be checksummed and figure out how many leaves
  * it would require to store the csums for that many bytes.

fs/btrfs/inode.c

@@ -3371,15 +3371,12 @@ static int check_data_csum(struct inode *inode, struct btrfs_bio *bbio,
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
 	u32 len = fs_info->sectorsize;
-	const u32 csum_size = fs_info->csum_size;
-	unsigned int offset_sectors;
 	u8 *csum_expected;
 	u8 csum[BTRFS_CSUM_SIZE];
 
 	ASSERT(pgoff + len <= PAGE_SIZE);
 
-	offset_sectors = bio_offset >> fs_info->sectorsize_bits;
-	csum_expected = ((u8 *)bbio->csum) + offset_sectors * csum_size;
+	csum_expected = btrfs_csum_ptr(fs_info, bbio->csum, bio_offset);
 
 	if (btrfs_check_sector_csum(fs_info, page, pgoff, csum, csum_expected))
 		goto zeroit;
@@ -8020,12 +8017,8 @@ static inline blk_status_t btrfs_submit_dio_bio(struct bio *bio,
 		if (ret)
 			goto err;
 	} else {
-		u64 csum_offset;
-
-		csum_offset = file_offset - dip->file_offset;
-		csum_offset >>= fs_info->sectorsize_bits;
-		csum_offset *= fs_info->csum_size;
-		btrfs_bio(bio)->csum = dip->csums + csum_offset;
+		btrfs_bio(bio)->csum = btrfs_csum_ptr(fs_info, dip->csums,
+						      file_offset - dip->file_offset);
 	}
 map:
 	ret = btrfs_map_bio(fs_info, bio, 0);