Re: [Xen-devel] VMX status report. Xen:26323 & Dom0:3.7.1

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.01.13 at 18:10, Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx> wrote:
> On Jan 10, 2013, at 3:57 AM, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> 
>>>>> On 10.01.13 at 08:51, "Ren, Yongjie" <yongjie.ren@xxxxxxxxx> wrote:
>>> New issue(1)
>>> ==============
>>> 1. sometimes live migration failed and reported call trace in dom0
>>>  http://bugzilla.xen.org/bugzilla/show_bug.cgi?id=1841 
>> 
>> For the failed allocation, the only obvious candidate appears to be
>> 
>>      err_array = kcalloc(m.num, sizeof(int), GFP_KERNEL);
>> 
>> which quite obviously can be of (almost) arbitrary size because
>> 
>>      nr_pages = m.num;
>>      if ((m.num <= 0) || (nr_pages > (LONG_MAX >> PAGE_SHIFT)))
>>              return -EINVAL;
>> 
>> really only checks for completely insane values.
>> 
>> This got introduced by Andres' "xen/privcmd: add PRIVCMD_MMAPBATCH_V2
>> ioctl" and is becoming worse with Mukesh's recent "xen: privcmd:
>> support autotranslated physmap guests", which added another
>> similar (twice as large) allocation in alloc_empty_pages().
> 
> Perhaps the err_array in this case, since alloc_empty_pages only happens for 
> auto translated dom0s.
> 
> Not familiar wether libxl changes (or is even capable of changing) 
> parameters of the migration code. But right now in libxc, mapping is done in 
> MAX_BATCH_SIZE batches, which are of size 1024. So we are talking about 1024 
> ints, which is *one* page.
> 
> So is really the kernel incapable of allocating one measly page?
> 
> This leads me to think that it might be gather_array, but that one would 
> allocate a grand total of two pages.

The log indicated a failed order-4 allocation though. So maybe not
that allocation after all (be the basically unbounded size here is a
problem in any case).

> In any case, both functions allocate arbitrary number of pages, and that is 
> the fundamental problem.
> 
> What is the approach in the forward ported kernel wrt to gather_array?

There's no gather_array there. It simply sets up things one page
at a time. (And you can always go and check linux-2.6.18-xen.hg).

Jan


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