f2fs: enhance multi-threads performance

Previously, f2fs_write_data_pages has a mutex, sbi->writepages, to serialize
data writes to maximize write bandwidth, while sacrificing multi-threads
performance.
Practically, however, multi-threads environment is much more important for
users. So this patch tries to remove the mutex.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1397,7 +1397,6 @@ static int f2fs_write_data_pages(struct address_space *mapping,
 {
 	struct inode *inode = mapping->host;
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
-	bool locked = false;
 	int ret;
 	long diff;
 
@@ -1418,13 +1417,7 @@ static int f2fs_write_data_pages(struct address_space *mapping,
 
 	diff = nr_pages_to_write(sbi, DATA, wbc);
 
-	if (!S_ISDIR(inode->i_mode)) {
-		mutex_lock(&sbi->writepages);
-		locked = true;
-	}
 	ret = write_cache_pages(mapping, wbc, __f2fs_writepage, mapping);
-	if (locked)
-		mutex_unlock(&sbi->writepages);
 
 	f2fs_submit_merged_bio(sbi, DATA, WRITE);
 

fs/f2fs/f2fs.h

@@ -613,7 +613,6 @@ struct f2fs_sb_info {
 	struct mutex cp_mutex;			/* checkpoint procedure lock */
 	struct rw_semaphore cp_rwsem;		/* blocking FS operations */
 	struct rw_semaphore node_write;		/* locking node writes */
-	struct mutex writepages;		/* mutex for writepages() */
 	wait_queue_head_t cp_wait;
 
 	struct inode_management im[MAX_INO_ENTRY];      /* manage inode cache */

fs/f2fs/super.c

@@ -1027,7 +1027,6 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->raw_super = raw_super;
 	sbi->raw_super_buf = raw_super_buf;
 	mutex_init(&sbi->gc_mutex);
-	mutex_init(&sbi->writepages);
 	mutex_init(&sbi->cp_mutex);
 	init_rwsem(&sbi->node_write);
 	clear_sbi_flag(sbi, SBI_POR_DOING);