mtime is older that atime on prl_fs partition

[MarcinK5]

Hi guys,

I wanted to use the folder sharing feature to share source code between my Mac OS X host & a linux VM which is used to compile all this stuff into an embedded linux system image using Buildroot.
Some of the packages use autotools to automatically detect when a rebuild is necessary.
The problem arises when a file's mtime & atime are not correctly updated, triggering a rebuild when it was not necessary.

I have tracked this down because that same code builds successfully on an ext4 partition.

Here's my test scenario: touch a file & inspect it's atime & mtime using stat. Then modify the contents of the file using nano & re-inspecting atime & mtime.

This is the result on an ext4 partition:

marcin@linux:~$ touch test
marcin@linux:~$ stat test

File: test
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: 802h/2050d Inode: 3934465 Links: 1
Access: (0664/-rw-rw-r--) Uid: ( 1000/ marcin) Gid: ( 1000/ marcin)
Access: 2021-09-21 23:21:21.633600878 +0200
Modify: 2021-09-21 23:21:21.633600878 +0200
Change: 2021-09-21 23:21:21.633600878 +0200
Birth: 2021-09-21 23:21:21.633600878 +0200​

marcin@linux:~$ nano test
marcin@linux:~$ stat test

File: test
Size: 2 Blocks: 8 IO Block: 4096 regular file
Device: 802h/2050d Inode: 3934465 Links: 1
Access: (0664/-rw-rw-r--) Uid: ( 1000/ marcin) Gid: ( 1000/ marcin)
Access: 2021-09-21 23:21:33.397639747 +0200
Modify: 2021-09-21 23:21:36.637650465 +0200
Change: 2021-09-21 23:21:36.637650465 +0200
Birth: 2021-09-21 23:21:21.633600878 +0200

​

And now on a prl_fs partition:

marcin@linux:/media/psf/$ touch test
marcin@linux:/media/psf/$ stat test

File: test
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: 39h/57d Inode: 309064 Links: 1
Access: (0664/-rw-rw-r--) Uid: ( 1000/ marcin) Gid: ( 1000/ marcin)
Access: 2021-09-21 23:20:03.000000000 +0200
Modify: 2021-09-21 23:20:03.000000000 +0200
Change: 2021-09-21 23:20:03.000000000 +0200
Birth: -​

marcin@linux:/media/psf/$ nano test
marcin@linux:/media/psf/$ stat test

File: test
Size: 2 Blocks: 8 IO Block: 4096 regular file
Device: 39h/57d Inode: 309064 Links: 1
Access: (0664/-rw-rw-r--) Uid: ( 1000/ marcin) Gid: ( 1000/ marcin)
Access: 2021-09-21 23:20:29.000000000 +0200
Modify: 2021-09-21 23:20:28.000000000 +0200
Change: 2021-09-21 23:20:28.000000000 +0200
Birth: -​

As you can see, only atime was updated on the prf_fs partition while the contents have changed!

Am I the only one with this issue?
Can you reproduce this on your end?

I have glanced over the source code and it seems it would be possible to patch this but I'm having trouble figuring out where & what to patch

Thanks!

 

[MarcinK5]

EDIT: mtime & atime are indeed both modified but mtime is older than atime.
Is this due to the driver reading that file & deciding if it needs to be synchronized back to the host?