Re: [PATCH v2] fix Remus failover regression

[Hongyang Yang <yanghy@cn.fujitsu.com>]

Hi Andrew,

On 07/28/2014 05:24 PM, Andrew Cooper wrote:
> On 28/07/14 05:03, Yang Hongyang wrote:
>> commit: c2ba706c
>> tools/libxc: goto correct label on error paths by Andrew Cooper
>> broke Remus in Xen 4.4 or earlier versions that has this commit
>> backported.
>
> My appologies for breaking Remus. (it just goes to show how fragile this
> code is).
>
>>
>> With Remus, this jump essentially discards the current incomplete
>> checkpoint received by the backup and restore backup from the
>> last complete checkpoint.
>> This is required for Remus to work and this does not break live
>> migration.
>> It has been around since Xen 4.0.
>
> However, it is a genuine bugfix for regular migration, so simply
> reverting it as this patch does is not appropriate.
>
> For regular migration, you absolutely have to goto out; on a failure
> otherwise the finish code will run and declare the migration a success
> despite only having half a domain restored.

I think regular migration shouldn't run into this path (see what I commented
in v1), but I agree that add a check will be better.

>
> You need something like:
>
> if ( !checkpointed_stream )
>      goto err;
>
> /* Remus comment */
> goto finish;
>
> to deal with the different error handing requirements of remus and
> regular streams.
>
> ~Andrew
>
>>
>> CC: Ian Jackson <ian.jackson@eu.citrix.com>
>> CC: Ian Campbell <ian.campbell@citrix.com>
>> CC: Andrew Cooper <andrew.cooper3@citrix.com>
>> CC: Shriram Rajagopalan <rshriram@cs.ubc.ca>
>> Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
>> ---
>>   tools/libxc/xc_domain_restore.c | 13 +++++++++++--
>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/libxc/xc_domain_restore.c b/tools/libxc/xc_domain_restore.c
>> index e73e0a2..b9a56d5 100644
>> --- a/tools/libxc/xc_domain_restore.c
>> +++ b/tools/libxc/xc_domain_restore.c
>> @@ -1783,20 +1783,29 @@ int xc_domain_restore(xc_interface *xch, int io_fd, uint32_t dom,
>>
>>       if ( pagebuf_get(xch, ctx, &pagebuf, io_fd, dom) ) {
>>           PERROR("error when buffering batch, finishing");
>> -        goto out;
>> +        /*
>> +         * Remus: discard the current incomplete checkpoint and restore
>> +         * backup from the last complete checkpoint.
>> +         */
>> +        goto finish;
>>       }
>>       memset(&tmptail, 0, sizeof(tmptail));
>>       tmptail.ishvm = hvm;
>>       if ( buffer_tail(xch, ctx, &tmptail, io_fd, max_vcpu_id, vcpumap,
>>                        ext_vcpucontext, vcpuextstate_size) < 0 ) {
>>           ERROR ("error buffering image tail, finishing");
>> -        goto out;
>> +        /*
>> +         * Remus: discard the current incomplete checkpoint and restore
>> +         * backup from the last complete checkpoint.
>> +         */
>> +        goto finish;
>>       }
>>       tailbuf_free(&tailbuf);
>>       memcpy(&tailbuf, &tmptail, sizeof(tailbuf));
>>
>>       goto loadpages;
>>
>> +  /* With Remus: restore from last complete checkpoint */
>>     finish:
>>       if ( hvm )
>>           goto finish_hvm;
>
> .
>

-- 
Thanks,
Yang.