[PATCH RFC 06/17] ext4: add ext4_block_bitmap_csum_set_range() for incremental checksum update

[Baokun Li <libaokun@linux.alibaba.com>]

Add a helper function ext4_block_bitmap_csum_set_range() that updates
the block bitmap checksum using crc32c_flip_range() for incremental CRC
calculation. Unlike ext4_block_bitmap_csum_set() which re-scans the
entire bitmap buffer, this function efficiently computes the CRC delta
for a range of flipped bits, avoiding the cost of a full CRC
recalculation.

Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/ext4/bitmap.c | 24 ++++++++++++++++++++++++
 fs/ext4/ext4.h   |  3 +++
 2 files changed, 27 insertions(+)

diff --git a/fs/ext4/bitmap.c b/fs/ext4/bitmap.c
index 46affc9e80ca..00b0a3c74859 100644
--- a/fs/ext4/bitmap.c
+++ b/fs/ext4/bitmap.c
@@ -110,3 +110,27 @@ void ext4_block_bitmap_csum_set(struct super_block *sb,
 	csum = ext4_chksum(sbi->s_csum_seed, (__u8 *)bh->b_data, sz);
 	ext4_block_bitmap_csum_store(sb, gdp, csum);
 }
+
+/*
+ * Update block bitmap checksum using incremental CRC calculation.
+ *
+ * This function assumes that ALL bits in the range [offset, offset+len)
+ * have been flipped (XORed with 1). It uses crc32c_flip_range() to
+ * efficiently compute the CRC delta without re-scanning the entire bitmap.
+ * The csum_seed cancels out in the XOR delta, so it is not needed here.
+ */
+void ext4_block_bitmap_csum_set_range(struct super_block *sb,
+				      struct ext4_group_desc *gdp,
+				      ext4_grpblk_t offset, ext4_grpblk_t len)
+{
+	__u32 new_csum, old_csum;
+
+	if (!ext4_has_feature_metadata_csum(sb))
+		return;
+
+	old_csum = ext4_block_bitmap_csum_get(sb, gdp);
+	new_csum = crc32c_flip_range(old_csum, EXT4_CLUSTERS_PER_GROUP(sb),
+				     offset, len);
+
+	ext4_block_bitmap_csum_store(sb, gdp, new_csum);
+}
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 94283a991e5c..c423a9a04047 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2770,6 +2770,9 @@ int ext4_inode_bitmap_csum_verify(struct super_block *sb,
 void ext4_block_bitmap_csum_set(struct super_block *sb,
 				struct ext4_group_desc *gdp,
 				struct buffer_head *bh);
+void ext4_block_bitmap_csum_set_range(struct super_block *sb,
+				      struct ext4_group_desc *gdp,
+				      ext4_grpblk_t offset, ext4_grpblk_t len);
 int ext4_block_bitmap_csum_verify(struct super_block *sb,
 				  struct ext4_group_desc *gdp,
 				  struct buffer_head *bh);
-- 
2.43.7