fs/9p: inode blocks show error in fscache mode

Hi all,

I test a scenario that will cause the difference of inode blocks between client and host, the scenario as follows:

Precondition:
1) use VirtFS(virtio-9p) to connect guest and host.
2) 9p dir in guest is /mnt/9p, host is /9p-host.
3) server fs is ext4 and block size is 4096.

Test steps:
1) on the client(guest)

# touch /mnt/9p/test/file
# dd if=/dev/zero of=/mnt/9p/test/file bs=1 count43456 seek02528 conv=notrunc

2) on the client(guest)

# stat /mnt/9p/test/file
the file's blocks is 4582 blocks(block size is 512)

3) on the server(host)

# stat /9p-host/test/file
the file's blocks is 2040 blocks(block size is 512)

Cause analysis:
Because the file is sparse file, so in function v9fs_write_end will update inode blocks according to difference between last_pos and inode_size, only when last_pos is larger than the inode_size, then it update the blocks and inode_size, the operation is not fit the sparse file.

Currently I want to call v9fs_invalidate_inode_attr to invalidate inode, but it will influence the performance, so I don't have a good solution.

Please advise. Thanks in advance!

Best regards,
Yiwen