Re: [Xen-devel] [PATCH] x86: use 'dom0_mem' to limit the number of pages for dom0

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 23/08/11 10:01, Jan Beulich wrote:
>>>> On 15.08.11 at 20:08, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
>> x86: use 'dom0_mem' to limit the number of pages for dom0
>>
>> Use the 'dom0_mem' command line option to set the maximum number of
>> pages for dom0.  dom0 can use then use the XENMEM_maximum_reservation
>> memory op to automatically find this limit and reduce the size of any
>> page tables etc.
>>
>> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
>>
>> diff -r 8d6edc3d26d2 -r 1ec6c392d40f xen/arch/x86/domain_build.c
>> --- a/xen/arch/x86/domain_build.c    Sat Aug 13 10:14:58 2011 +0100
>> +++ b/xen/arch/x86/domain_build.c    Mon Aug 15 11:42:37 2011 +0100
>> @@ -254,6 +254,8 @@ static unsigned long __init compute_dom0
>>      }
>>  #endif
>>  
>> +    d->max_pages = min(max_pages, avail);
> 
> Only having noticed this hypervisor side change in the patch set as it
> now got applied to -unstable already, I have to object to this: It is
> not memory hotplug compatible (i.e. Dom0, at least with a non-pvops
> kernel, could have got ballooned up past the original limit once memory
> got added to the system, whereas now you limit it to what was there
> at boot time). If anything, this should be set to just max_pages, not
> considering the amount of available memory at all.
> 
> Besides that the assignment needs to be saturating, as struct domain's
> max_pages member is only "unsigned int", whereas the local variables
> are all "unsigned long". So in the end this should really be an equivalent
> of
> 
>     d->max_pages = min(max_pages, UINT_MAX);

Yeah, this makes sense.  I'll send another patch tomorrow.

David

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