f2fs: optimize f2fs_overwrite_io() for f2fs_iomap_begin

When overwriting already allocated blocks, f2fs_iomap_begin() calls
f2fs_overwrite_io() to check block mappings. However,
f2fs_overwrite_io() iterates through all mapped blocks in the range,
which can be inefficient for fragmented files with large I/O requests.

This patch optimizes f2fs_overwrite_io() by adding a 'check_first'
parameter and introducing __f2fs_overwrite_io() helper. When called from
f2fs_iomap_begin(), we only check the first mapping to determine if the
range is already allocated, which is sufficient for setting
map.m_may_create.

This optimization significantly reduces the number of f2fs_map_blocks()
calls in f2fs_overwrite_io() when called from f2fs_iomap_begin(),
especially for fragmented files with large I/O requests.

Cc: stable@kernel.org
Fixes: 351bc76 ("f2fs: optimize f2fs DIO overwrites")
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: Sunmin Jeong <s_min.jeong@samsung.com>
Signed-off-by: Yeongjin Gil <youngjin.gil@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: YeongjinGil

fs/f2fs/data.c

@@ -1865,7 +1865,8 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map, int flag)
 	return err;
 }
 
-bool f2fs_overwrite_io(struct inode *inode, loff_t pos, size_t len)
+static bool __f2fs_overwrite_io(struct inode *inode, loff_t pos, size_t len,
+				bool check_first)
 {
 	struct f2fs_map_blocks map;
 	block_t last_lblk;
@@ -1887,10 +1888,17 @@ bool f2fs_overwrite_io(struct inode *inode, loff_t pos, size_t len)
 		if (err || map.m_len == 0)
 			return false;
 		map.m_lblk += map.m_len;
+		if (check_first)
+			break;
 	}
 	return true;
 }
 
+bool f2fs_overwrite_io(struct inode *inode, loff_t pos, size_t len)
+{
+	return __f2fs_overwrite_io(inode, pos, len, false);
+}
+
 static int f2fs_xattr_fiemap(struct inode *inode,
 				struct fiemap_extent_info *fieinfo)
 {
@@ -4487,7 +4495,7 @@ static int f2fs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	 * f2fs_map_lock and f2fs_balance_fs are not necessary.
 	 */
 	if ((flags & IOMAP_WRITE) &&
-		!f2fs_overwrite_io(inode, offset, length))
+		!__f2fs_overwrite_io(inode, offset, length, true))
 		map.m_may_create = true;
 
 	err = f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_DIO);