Re: [Qemu-devel] [PATCH 1/2] qemu-ga: execute script to quiesce the guest on fsfreeze-freeze/thaw

[Tomoki Sekiyama]

Hi Eric, thanks for your review.

On 2012/11/09 3:38, Eric Blake wrote:
> On 11/08/2012 05:05 AM, Tomoki Sekiyama wrote:
> 
> [Recoding to UTF-8, as ISO-2022-JP is not universally installed these
> days - you may want to reconsider your mailer's defaults]

Now this should be UTF-8, sorry.

>> To use the online disk snapshot for online-backup, application-level
>> consistency of the snapshot image is required. However, currently the
>> guest agent can provide only filesystem-level consistency, and the
>> snapshot may contain dirty data, for example, incomplete transactions.
>> This patch provides the opportunity to quiesce applications before
>> snapshot is taken.
>>
>> When the qemu-ga receives fsfreeze-freeze command, the script specified
>> in --fsfreeze-script option is executed with "freeze" argument before the
>> filesystem is frozen. For fsfreeze-thaw command, the script is executed
>> with "thaw" argument after the filesystem is thawed.
>>
>> Signed-off-by: Tomoki Sekiyama <address@hidden>
>> ---
> 
>> @@ -396,6 +397,34 @@ GuestFsfreezeStatus qmp_guest_fsfreeze_status(Error 
>> **err)
>>      return GUEST_FSFREEZE_STATUS_THAWED;
>>  }
>>  
>> +int execute_fsfreeze_script(const char *arg)
>> +{
>> +    int ret = -1;
>> +    const char *fsfreeze_script;
>> +    char *cmdline;
>> +    struct stat st;
>> +
>> +    fsfreeze_script = ga_fsfreeze_script(ga_state);
>> +    if (fsfreeze_script && stat(fsfreeze_script, &st) == 0) {
>> +        if (S_ISREG(st.st_mode) && (st.st_mode & S_IXUSR)) {
> 
> Is it any simpler to use access(fsfreeze_script, X_OK) to check if the
> script exists and is executable?

OK, I will use access() here.

<snip>
>> +                ret = system(cmdline);
> 
> ...system() is not required to be thread-safe, but we should assume that
> qemu-ga is multi-threaded, and therefore we should not use system.
> Besides executing things via an extra layer of shell opens doors for
> further problems; for example, if the user starts qemu-ga with
> --fsfreeze-script '/path/with spaces/script', your command line is
> horribly broken when passed through the shell.  It would be much better
> to directly fork() and exec() the script ourselves instead of relying on
> system().

I will use fork()/exec() method instead of system(), as in shutdown,
so I can remove malloc/free.

>> +            if (ret > 0) {
>> +                g_warning("fsfreeze script failed with status=%d", ret);
> 
> This is a potentially misleading message; you should be using macros
> such as WEXITSTATUS when interpreting the result of system(), since not
> all systems return exit status 1 in the same bit position.

OK. I will refine error messages.

> The idea of having the freeze and thaw actions hook out to
> user-specified actions for additional steps seems nice, but this patch
> series needs a lot more work.

Regards,
-- 
Tomoki Sekiyama <address@hidden>
Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory