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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Oct 10, 2013 at 03:53:08PM +0800, Gu Zheng wrote:
> 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?
> 
The purpose of GUP here is to bring page from swap, the page itself is
never used directly by async pf code. The page is used when guest
accesses it next time, but that code path does its own GUP.

--
			Gleb.