f2fs: fix to update time in lazytime mode

generic/018 reports an inconsistent status of atime, the
testcase is as below:
- open file with O_SYNC
- write file to construct fraged space
- calc md5 of file
- record {a,c,m}time
- defrag file --- do nothing
- umount & mount
- check {a,c,m}time

The root cause is, as f2fs enables lazytime by default, atime
update will dirty vfs inode, rather than dirtying f2fs inode (by set
with FI_DIRTY_INODE), so later f2fs_write_inode() called from VFS will
fail to update inode page due to our skip:

f2fs_write_inode()
	if (is_inode_flag_set(inode, FI_DIRTY_INODE))
		return 0;

So eventually, after evict(), we lose last atime for ever.

To fix this issue, we need to check whether {a,c,m,cr}time is
consistent in between inode cache and inode page, and only skip
f2fs_update_inode() if f2fs inode is not dirty and time is
consistent as well.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -2704,6 +2704,20 @@ static inline void clear_file(struct inode *inode, int type)
 	f2fs_mark_inode_dirty_sync(inode, true);
 }
 
+static inline bool f2fs_is_time_consistent(struct inode *inode)
+{
+	if (!timespec64_equal(F2FS_I(inode)->i_disk_time, &inode->i_atime))
+		return false;
+	if (!timespec64_equal(F2FS_I(inode)->i_disk_time + 1, &inode->i_ctime))
+		return false;
+	if (!timespec64_equal(F2FS_I(inode)->i_disk_time + 2, &inode->i_mtime))
+		return false;
+	if (!timespec64_equal(F2FS_I(inode)->i_disk_time + 3,
+						&F2FS_I(inode)->i_crtime))
+		return false;
+	return true;
+}
+
 static inline bool f2fs_skip_inode_update(struct inode *inode, int dsync)
 {
 	bool ret;
@@ -2721,14 +2735,7 @@ static inline bool f2fs_skip_inode_update(struct inode *inode, int dsync)
 			i_size_read(inode) & ~PAGE_MASK)
 		return false;
 
-	if (!timespec64_equal(F2FS_I(inode)->i_disk_time, &inode->i_atime))
-		return false;
-	if (!timespec64_equal(F2FS_I(inode)->i_disk_time + 1, &inode->i_ctime))
-		return false;
-	if (!timespec64_equal(F2FS_I(inode)->i_disk_time + 2, &inode->i_mtime))
-		return false;
-	if (!timespec64_equal(F2FS_I(inode)->i_disk_time + 3,
-						&F2FS_I(inode)->i_crtime))
+	if (!f2fs_is_time_consistent(inode))
 		return false;
 
 	down_read(&F2FS_I(inode)->i_sem);

fs/f2fs/inode.c

@@ -615,7 +615,11 @@ int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
 			inode->i_ino == F2FS_META_INO(sbi))
 		return 0;
 
-	if (!is_inode_flag_set(inode, FI_DIRTY_INODE))
+	/*
+	 * atime could be updated without dirtying f2fs inode in lazytime mode
+	 */
+	if (f2fs_is_time_consistent(inode) &&
+		!is_inode_flag_set(inode, FI_DIRTY_INODE))
 		return 0;
 
 	if (!f2fs_is_checkpoint_ready(sbi))