bug#43062: --expose in vm does not reflect file modifications in guest

bug#43062: --expose in vm does not reflect file modifications in guest

[elaexuotee--- via Bug reports for GNU Guix]

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## Overview

When using --expose to mirror a path between host and guest, the guest mirror
fails to reflect file modifications from the host. However, file creation and
deletion are correctly propogated.

To pick up file modifications in the guest, it is sufficient to remount
mirroring 9p filesystem.

Is this behaviour expected?


## Reproduction

The following should be sufficient to reproduce the issue:

Create a container and expose a path:

    host$ guix system --expose /some/path vm.scm
    /gnu/store/<hash>-run-vm.sh

Spin up the vm:

    host$ /gnu/store/<hash>-run-vm.sh

From the host, create a new file under /some/path, and check that the guest
sees this file:

    host$ touch /some/path/test
    guest$ cat /some/path/test
    <file is empty>

Now change the contents of this file on the host, and verify that the guest
does not see the change:

    host$ echo foo >/some/path/test
    guest$ cat /some/path/test
    <file is empty>

Finally, remount the filesystem at /some/path, and see that the guest now picks
up the changes:

    guest$ sudo mount -o remount,ro /some/path
    guest$ cat /some/path/test
    foo


## Version Information

    $ guix describe
    Generation 123	Aug 25 2020 23:19:12	(current)
      guix 253fcfe
        repository URL: https://git.savannah.gnu.org/git/guix.git
        branch: master
        commit: 253fcfe6fec8fb9d70cde8623fe562dc3ca67262

    $ cat vm.scm
    (use-modules (gnu))
    (use-service-modules networking ssh)
    (use-package-modules admin linux ncurses tmux)
    
    (operating-system
      (host-name "fmadio")
      (timezone "Asia/Tokyo")
      (locale "en_US.utf8")
    
      (bootloader (bootloader-configuration (bootloader #f) (target #f)))
      (kernel linux-libre-4.9)
      (file-systems %base-file-systems)
      (users (cons (user-account
                     (name "x")
                     (password (crypt "x" "Jr1er07l0lOUQ95GQLijow=="))
                     (group "users")
                     (supplementary-groups '("wheel")))
                   %base-user-accounts))
      (packages (cons* ncurses tcpdump tmux %base-packages))
      (services (cons* (service dhcp-client-service-type)
                       (service openssh-service-type)
                       %base-services)))


## Notes

In the above, I am running linux-libre@4.9 in the guest, but another user on
#guix confirmed the issue with linux-libre@5.8. The same user reported seeing
the following after modifying the host file and remounting in the guest:

    guest$ cat /some/path/test
    [   49.263620] FS-Cache: Duplicate cookie detected
    [   49.263644] FS-Cache: O-cookie c=00000000fe189610 [p=000000004224ad86 fl=226 nc=0 na=1]
    [   49.263664] FS-Cache: O-cookie d=0000000023080181 n=00000000825c3154
    [   49.263680] FS-Cache: O-key=[8] 'c3f51c0500000000'
    [   49.263695] FS-Cache: N-cookie c=00000000c11e31c7 [p=000000004224ad86 fl=2 nc=0 na=1]
    [   49.263715] FS-Cache: N-cookie d=0000000023080181 n=00000000dad562d4
    [   49.263731] FS-Cache: N-key=[8] 'c3f51c0500000000'
    foo

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bug#43062: --expose in vm does not reflect file modifications in guest

[Ludovic Courtès]

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Hi,

elaexuotee--- via Bug reports for GNU Guix <bug-guix@gnu.org> skribis:

> When using --expose to mirror a path between host and guest, the guest mirror
> fails to reflect file modifications from the host. However, file creation and
> deletion are correctly propogated.
>
> To pick up file modifications in the guest, it is sufficient to remount
> mirroring 9p filesystem.
>
> Is this behaviour expected?

I believe this comes from the “cache=loose” 9p mount option added in
commit e0d96774dd48c29ccc4c90fea1f8f71850ab0879.

Does the patch below help?

Chris, what do you think?  How would this affect the performance issues
that led to e0d96774dd48c29ccc4c90fea1f8f71850ab0879?

Thanks,
Ludo’.


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diff --git a/gnu/system/vm.scm b/gnu/system/vm.scm
index 861f2a427a..80a8618729 100644
--- a/gnu/system/vm.scm
+++ b/gnu/system/vm.scm
@@ -699,7 +699,8 @@ of the GNU system as described by OS."
        (device (file-system->mount-tag source))
        (type "9p")
        (flags (if writable? '() '(read-only)))
-       (options "trans=virtio,cache=loose")
+       (options (string-append "trans=virtio"
+                               (if writable? "" ",cache=loose")))
        (check? #f)
        (create-mount-point? #t)))))
 

bug#43062: --expose in vm does not reflect file modifications in guest

[elaexuotee--- via Bug reports for GNU Guix]

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> I believe this comes from the “cache=loose” 9p mount option added in
> commit e0d96774dd48c29ccc4c90fea1f8f71850ab0879.
> 
> Does the patch below help?

Yes, when no caching behaviour is specified in the guest, the problem
disappears. I also tried cache=fscache and see the exact same problem as with
cache=loose.


Cheers!

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bug#43062: --expose in vm does not reflect file modifications in guest

[Christopher Baines]

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Ludovic Courtès <ludo@gnu.org> writes:

> Hi,
>
> elaexuotee--- via Bug reports for GNU Guix <bug-guix@gnu.org> skribis:
>
>> When using --expose to mirror a path between host and guest, the guest mirror
>> fails to reflect file modifications from the host. However, file creation and
>> deletion are correctly propogated.
>>
>> To pick up file modifications in the guest, it is sufficient to remount
>> mirroring 9p filesystem.
>>
>> Is this behaviour expected?
>
> I believe this comes from the “cache=loose” 9p mount option added in
> commit e0d96774dd48c29ccc4c90fea1f8f71850ab0879.
>
> Does the patch below help?
>
> Chris, what do you think?  How would this affect the performance issues
> that led to e0d96774dd48c29ccc4c90fea1f8f71850ab0879?

Caching only for readonly filesystems sounds fine, I think I was only
thinking about the store for e0d96774dd48c29ccc4c90fea1f8f71850ab0879.

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bug#43062: --expose in vm does not reflect file modifications in guest

[Ludovic Courtès]

Hello!

Christopher Baines <mail@cbaines.net> skribis:

> Ludovic Courtès <ludo@gnu.org> writes:
>
>> Hi,
>>
>> elaexuotee--- via Bug reports for GNU Guix <bug-guix@gnu.org> skribis:
>>
>>> When using --expose to mirror a path between host and guest, the guest mirror
>>> fails to reflect file modifications from the host. However, file creation and
>>> deletion are correctly propogated.
>>>
>>> To pick up file modifications in the guest, it is sufficient to remount
>>> mirroring 9p filesystem.
>>>
>>> Is this behaviour expected?
>>
>> I believe this comes from the “cache=loose” 9p mount option added in
>> commit e0d96774dd48c29ccc4c90fea1f8f71850ab0879.
>>
>> Does the patch below help?
>>
>> Chris, what do you think?  How would this affect the performance issues
>> that led to e0d96774dd48c29ccc4c90fea1f8f71850ab0879?
>
> Caching only for readonly filesystems sounds fine, I think I was only
> thinking about the store for e0d96774dd48c29ccc4c90fea1f8f71850ab0879.

Alright, I went ahead and did that in
7eeb78157d3d0267ce4a4ea38ff56a2c4246c11b.

Thanks!

Ludo’.