[PATCH RFC 09/17] ext4: add ext4_inode_bitmap_csum_set_fast() for incremental checksum update

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

Add a helper function ext4_inode_bitmap_csum_set_fast() that uses
crc32c_flip_range() to incrementally update the inode bitmap checksum
after flipping a single bit at the given offset. This avoids a full
bitmap CRC rescan, computing the CRC delta in O(log N) time.

Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/ext4/bitmap.c | 23 +++++++++++++++++++++++
 fs/ext4/ext4.h   |  3 +++
 2 files changed, 26 insertions(+)

diff --git a/fs/ext4/bitmap.c b/fs/ext4/bitmap.c
index 008acc439301..ea47ca0d7046 100644
--- a/fs/ext4/bitmap.c
+++ b/fs/ext4/bitmap.c
@@ -71,6 +71,29 @@ void ext4_inode_bitmap_csum_set(struct super_block *sb,
 	ext4_inode_bitmap_csum_store(sb, gdp, csum);
 }
 
+/*
+ * Update inode bitmap checksum for a single flipped bit.
+ *
+ * Use crc32c_flip_range() to incrementally update the checksum after
+ * flipping the bit at @offset, avoiding a full bitmap CRC rescan.
+ * The csum_seed cancels out in the XOR delta, so it is not needed here.
+ */
+void ext4_inode_bitmap_csum_set_fast(struct super_block *sb,
+				     struct ext4_group_desc *gdp,
+				     ext4_grpblk_t offset)
+{
+	__u32 new_csum, old_csum;
+
+	if (!ext4_has_feature_metadata_csum(sb))
+		return;
+
+	old_csum = ext4_inode_bitmap_csum_get(sb, gdp);
+	new_csum = crc32c_flip_range(old_csum, EXT4_INODES_PER_GROUP(sb),
+				     offset, 1);
+
+	ext4_inode_bitmap_csum_store(sb, gdp, new_csum);
+}
+
 static inline __u32 ext4_block_bitmap_csum_get(struct super_block *sb,
 					       struct ext4_group_desc *gdp)
 {
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index c423a9a04047..e6739d5af490 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2764,6 +2764,9 @@ extern unsigned int ext4_count_free(char *bitmap, unsigned numchars);
 void ext4_inode_bitmap_csum_set(struct super_block *sb,
 				struct ext4_group_desc *gdp,
 				struct buffer_head *bh);
+void ext4_inode_bitmap_csum_set_fast(struct super_block *sb,
+				     struct ext4_group_desc *gdp,
+				     ext4_grpblk_t offset);
 int ext4_inode_bitmap_csum_verify(struct super_block *sb,
 				  struct ext4_group_desc *gdp,
 				  struct buffer_head *bh);
-- 
2.43.7