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

[Gu Zheng]

Hi Gleb,

On 10/10/2013 03:15 PM, Gleb Natapov wrote:

> On Thu, Oct 10, 2013 at 03:05:58PM +0800, chai wen wrote:
>> 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.
>>
> Should be extremely easy. Drop FOLL_GET from GUP in async_pf_execute().

One lower question, why pinning page is not necessary here?

Thanks,
Gu

> 
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