Re: [RFC/query] kvm async_pf anon pined pages migration

[chai wen <chaiw.fnst@cn.fujitsu.com>]

On 10/08/2013 03:39 PM, Gleb Natapov wrote:
> On Tue, Oct 08, 2013 at 02:58:22PM +0800, chai wen wrote:
>> On 10/02/2013 12:04 AM, chaiwen wrote:
>>> On 09/30/2013 08:51 PM, Gleb Natapov wrote:
>>>> On Mon, Sep 30, 2013 at 06:03:07PM +0800, chai wen wrote:
>>>>> Hi all
>>>>>
>>>>> Async page fault in kvm currently pin user pages via get_user_pages.
>>>>> when doing page migration,the method can be found via
>>>>> page->mmapping->a_ops->migratepage to offline old pages and migrate to
>>>>> new pages. As to anonymous page there is no file mapping but a anon_vma.So
>>>>> the migration will fall back to some *default* migration method.Anon pages
>>>>> that have been pined in memory by some reasons could be failed in the migration
>>>>> processing because of some reasons like ref-count checking.
>>>>> (or I misunderstand some thing?)
>>>>>
>>>>> Now we want to make these anon pages in async_pf can be migrated, I try some
>>>>> ways.But there are still many problems. The following is one that replaceing
>>>>> the mapping of anon page arbitrarily and doing some thing based on it.
>>>>> Kvm-based virtual machine can works on this patch,but have no experience of
>>>>> offline pages because of the limitaion of resouces.I'll check it later.
>>>>>
>>>>> I don't know weather it is a right direction of this issue.
>>>>> All comments/criticize are welcomed.
>>>> The pinning is not mandatory and can (and probably should) be dropped, but
>>>> pinning that is done by async page faults is short lived. What problems
>>>> are you seeing that warrant the complexity of handling their migration?
>> Hi Gleb
>>
>> As to this issue, I still have some thing not very clear.
>> If pages pinning is successfully holding (although not mandatory) by
>> async page fault.
>> And at the same time page migration happens because of memory
>> hot-remove action.
>> It has 120*hz timeout setting in common page offline processing,
>> could it fail with
>> these async_pf pined pages migration ?
>> What's your opinion about this ?   If it may fail under this
>> circumstance, should we do
>> some thing on it ?
>>
> 120 seconds is more than enough time for pinning to go away, but as I
> said the pinning is not even necessary. Patch to remove it is welcomed.
Thank you for your clarification !  I've got it. we will still work on it.

>
> --
> 			Gleb.
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-- 
Regards

chai wen