Re: Populate-on-demand memory problem

["Jan Beulich" <JBeulich@novell.com>]

Keir,

with Dietmar having tested this successfully, is there anything that
keeps this from being applied to -unstable (and perhaps also 4.0.1)?

Jan

>>> On 27.07.10 at 15:10, George Dunlap <George.Dunlap@eu.citrix.com> wrote:
> Hmm, looks like I neglected to push a fix upstream.  Can you test it
> with the attached patch, and tell me if that fixes your problem?
> 
>  -George
> 
> On Tue, Jul 27, 2010 at 8:48 AM, Dietmar Hahn
> <dietmar.hahn@ts.fujitsu.com> wrote:
>> Hi list,
>>
>> we ported our system from Novel SLES11 using xen-3.3 to SLES11 SP1 using
>> xen-4.0 and ran into some trouble with the pod stuff.
>> We have a HVM guest and already used target_mem < max_mem on startup of
>> the guest.
>> With the new xen version we get
>> (XEN) p2m_pod_demand_populate: Out of populate-on-demand memory! tot_pages 
> 792792 pod_entries 800
>> I did some code revisions and looking at pod patches
>> (http://lists.xensource.com/archives/html/xen-devel/2008-12/msg01030.html)
>> to understand the behavior. We use the following configuration:
>> maxmem = 4096
>> memory = 3096
>> What I see is:
>>  - our guest boots with e820 map showing maxmem.
>>  - reading xenstore memory/target returns '3170304' means 3096MB, 792576 
> pages
>> Now our guest uses the target memory and gives back 1000MB via
>> hypervisor call XENMEM_decrease_reservation to the hypervisor.
>>
>> Later I try to map the complete domU memory into dom0 kernel space and here 
> I
>> get the 'Out of populate-on-demand memory' crash.
>>
>> As far as I understand (ignoring the p2m_pod_emergency_sweep)
>> - on populating a page
>>   - the page is taken from the pod cache
>>   - p2md->pod.count--
>>   - p2md->pod.entry_count--
>>   - page gets type p2m_ram_rw
>> - decreasing a page
>>   - p2md->pod.entry_count--
>>   - page gets type p2m_invalid
>>
>> So if the guest uses all the target memory and gave back all
>> the (maxmem-target) memory p2md->pod.count and p2md->pod.entry_count should be
>> zero.
>> I added some tracing in the hypervisor and see on start of the guest:
>> p2m_pod_set_cache_target: p2md->pod.count: 791264 tot_pages: 791744
>> This pod.count is lower then the target seen in the guest!
>> On the first call of p2m_pod_demand_populate() I can see
>> p2m_pod_demand_populate: p2md->pod.entry_count: 1048064 p2md->pod.count: 791264 
> tot_pages: 792792
>> So pod.entry_count=1048064 (4096MB) complies to maxmem but
>> pod.count=791264 is lower then the target memory in xenstore.
>>
>> Any help is welcome!
>> Thanks.
>> Dietmar.
>>
>> --
>> Company details: http://ts.fujitsu.com/imprint.html 
>>
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