f2fs: add un/freeze_fs into super_operations

This patch supports ioctl FIFREEZE and FITHAW to snapshot filesystem.
Before calling f2fs_freeze, all writers would be suspended and sync_fs
would be completed. So no f2fs has to do something.
Just background gc operation should be skipped due to generate dirty
nodes and data until unfreeze.

Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/gc.c

@@ -44,6 +44,11 @@ static int gc_thread_func(void *data)
 		if (kthread_should_stop())
 			break;
 
+		if (sbi->sb->s_writers.frozen >= SB_FREEZE_WRITE) {
+			wait_ms = GC_THREAD_MAX_SLEEP_TIME;
+			continue;
+		}
+
 		f2fs_balance_fs(sbi);
 
 		if (!test_opt(sbi, BG_GC))

fs/f2fs/inode.c

@@ -278,13 +278,15 @@ void f2fs_evict_inode(struct inode *inode)
 	if (inode->i_nlink || is_bad_inode(inode))
 		goto no_delete;
 
+	sb_start_intwrite(inode->i_sb);
 	set_inode_flag(F2FS_I(inode), FI_NO_ALLOC);
 	i_size_write(inode, 0);
 
 	if (F2FS_HAS_BLOCKS(inode))
 		f2fs_truncate(inode);
 
 	remove_inode_page(inode);
+	sb_end_intwrite(inode->i_sb);
 no_delete:
 	clear_inode(inode);
 }

fs/f2fs/super.c

@@ -143,6 +143,22 @@ int f2fs_sync_fs(struct super_block *sb, int sync)
 	return 0;
 }
 
+static int f2fs_freeze(struct super_block *sb)
+{
+	int err;
+
+	if (sb->s_flags & MS_RDONLY)
+		return 0;
+
+	err = f2fs_sync_fs(sb, 1);
+	return err;
+}
+
+static int f2fs_unfreeze(struct super_block *sb)
+{
+	return 0;
+}
+
 static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
 	struct super_block *sb = dentry->d_sb;
@@ -213,6 +229,8 @@ static struct super_operations f2fs_sops = {
 	.evict_inode	= f2fs_evict_inode,
 	.put_super	= f2fs_put_super,
 	.sync_fs	= f2fs_sync_fs,
+	.freeze_fs	= f2fs_freeze,
+	.unfreeze_fs	= f2fs_unfreeze,
 	.statfs		= f2fs_statfs,
 };