f2fs: invalidate block device page cache on umount

Neither F2FS nor VFS invalidates the block device page cache, which
results in reading stale metadata. An example scenario is shown below:

Terminal A                  Terminal B
mount /dev/vdb /mnt/f2fs
touch mx // ino = 4
sync
dump.f2fs -i 4 /dev/vdb// block on "[Y/N]"
                            touch mx2 // ino = 5
                            sync
                            umount /mnt/f2fs
                            dump.f2fs -i 5 /dev/vdb // block addr is 0

After umount, the block device page cache is not purged, causing
`dump.f2fs -i 5 /dev/vdb` to read stale metadata and see inode 5 with
block address 0.

Btrfs has encountered a similar issue before, the solution there was to
call sync_blockdev() and invalidate_bdev() when the device is closed:

mail-archive.com/[email protected]/msg54188.html

For the root user, the f2fs kernel calls sync_blockdev() on umount to
flush all cached data to disk, and f2fs-tools can release the page cache
by issuing ioctl(fd, BLKFLSBUF) when accessing the device. However,
non-root users are not permitted to drop the page cache, and may still
observe stale data.

This patch calls sync_blockdev() and invalidate_bdev() during umount to
invalidate the block device page cache, thereby preventing stale
metadata from being read.

Note that this may result in an extra sync_blockdev() call on the first
device, in both f2fs_put_super() and kill_block_super(). The second call
do nothing, as there are no dirty pages left to flush. It ensures that
non-root users do not observe stale data.

Signed-off-by: Yongpeng Yang <[email protected]>
Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>

Author: Yongpeng Yang

fs/f2fs/super.c

@@ -2088,6 +2088,12 @@ static void f2fs_put_super(struct super_block *sb)
 #if IS_ENABLED(CONFIG_UNICODE)
 	utf8_unload(sb->s_encoding);
 #endif
+	sync_blockdev(sb->s_bdev);
+	invalidate_bdev(sb->s_bdev);
+	for (i = 1; i < sbi->s_ndevs; i++) {
+		sync_blockdev(FDEV(i).bdev);
+		invalidate_bdev(FDEV(i).bdev);
+	}
 }
 
 int f2fs_sync_fs(struct super_block *sb, int sync)