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

[Gleb Natapov <gleb@redhat.com>]

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().

--
			Gleb.