On Thu, Jun 17, 2021 at 06:12:56PM +0800, Wang Yugui wrote:

the major/minor value of statx(kernel samples/vfs/test-statx.c) does not
match /usr/bin/stat. 

major/minor of statx result seems be truncated by something like
old_decode_dev()?

I've checked the code first, both stat/statx seem to be doing the same
thing.

[root@T640 vfs]# ./test-statx /ssd/
statx(/ssd/) = 0
results=1fff
  Size: 200             Blocks: 0          IO Block: 4096    directory
Device: 00:31           Inode: 256         Links: 1

So Device is 00:31, printed as two fields in hex.

Access: (0755/drwxr-xr-x)  Uid:     0   Gid:     0
Access: 2021-06-16 19:16:56.644344956+0800
Modify: 2021-05-06 16:14:33.676248229+0800
Change: 2021-05-06 16:14:33.676248229+0800
 Birth: 2020-11-18 14:03:35.324915316+0800
Attributes: 0000000000002000 (........ ........ ........ ........ ........ ..-..... ..?-.... .---.-..)
[root@T640 vfs]# stat /ssd/
  File: ‘/ssd/’
  Size: 200             Blocks: 0          IO Block: 4096   directory
Device: 31h/49d Inode: 256         Links: 1

And here it's also 31, the 'h' suffix means it's hexadecimal and "49d"
si the same value in decimal.

Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2021-06-16 19:16:56.644344956 +0800
Modify: 2021-05-06 16:14:33.676248229 +0800
Change: 2021-05-06 16:14:33.676248229 +0800
 Birth: -

vfat output sample:
[root@T640 vfs]# ./test-statx /boot/efi/
statx(/boot/efi/) = 0
results=17ff
  Size: 4096            Blocks: 8          IO Block: 4096    directory
Device: 08:01           Inode: 1           Links: 3

08:01

Access: (0700/drwx------)  Uid:     0   Gid:     0
Access: 1970-01-01 08:00:00.000000000+0800
Modify: 1970-01-01 08:00:00.000000000+0800
Change: 1970-01-01 08:00:00.000000000+0800
Attributes: 0000000000002000 (........ ........ ........ ........ ........ ..-..... ..?-.... ........)
[root@T640 vfs]# stat /boot/efi/
  File: ‘/boot/efi/’
  Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: 801h/2049d      Inode: 1           Links: 3

801h == 0801h the same value, so it's just a matter of formatting the
output and the values are indeed the same.

If you change the format in test-statx.c to  "%2xh/%2dd" and the value
to "(stx->stx_dev_major << 8) + stx->stx_dev_minor", the output is
exactly the same.