f2fs: optimize f2fs DIO overwrites

this is unnecessary when we know we are overwriting already allocated
blocks and the overhead of starting a transaction can be significant
especially for multithreaded workloads doing small writes.

Signed-off-by: Yohan Joung <yohan.joung@sk.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: Yohan Joung

fs/f2fs/data.c

@@ -4181,7 +4181,13 @@ static int f2fs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	map.m_next_pgofs = &next_pgofs;
 	map.m_seg_type = f2fs_rw_hint_to_seg_type(F2FS_I_SB(inode),
 						inode->i_write_hint);
-	if (flags & IOMAP_WRITE)
+
+	/*
+	 * If the blocks being overwritten are already allocated,
+	 * f2fs_map_lock and f2fs_balance_fs are not necessary.
+	 */
+	if ((flags & IOMAP_WRITE) &&
+		!f2fs_overwrite_io(inode, offset, length))
 		map.m_may_create = true;
 
 	err = f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_DIO);