Re: [RFC] respect the referenced bit of KVM guest pages?

[KOSAKI Motohiro]

> On Tue, Aug 18, 2009 at 07:00:48PM +0800, Minchan Kim wrote:
> > On Tue, Aug 18, 2009 at 7:00 PM, Wu Fengguang<fengguang.wu@xxxxxxxxx> wrote:
> > > On Tue, Aug 18, 2009 at 05:52:47PM +0800, Minchan Kim wrote:
> > >> On Tue, 18 Aug 2009 17:31:19 +0800
> > >> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> > >>
> > >> > On Tue, Aug 18, 2009 at 12:17:34PM +0800, Minchan Kim wrote:
> > >> > > On Tue, 18 Aug 2009 10:34:38 +0800
> > >> > > Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> > >> > >
> > >> > > > Minchan,
> > >> > > >
> > >> > > > On Mon, Aug 17, 2009 at 10:33:54PM +0800, Minchan Kim wrote:
> > >> > > > > On Sun, Aug 16, 2009 at 8:29 PM, Wu Fengguang<fengguang.wu@xxxxxxxxx> wrote:
> > >> > > > > > On Sun, Aug 16, 2009 at 01:15:02PM +0800, Wu Fengguang wrote:
> > >> > > > > >> On Sun, Aug 16, 2009 at 11:53:00AM +0800, Rik van Riel wrote:
> > >> > > > > >> > Wu Fengguang wrote:
> > >> > > > > >> > > On Fri, Aug 07, 2009 at 05:09:55AM +0800, Jeff Dike wrote:
> > >> > > > > >> > >> Side question -
> > >> > > > > >> > >> ÂIs there a good reason for this to be in shrink_active_list()
> > >> > > > > >> > >> as opposed to __isolate_lru_page?
> > >> > > > > >> > >>
> > >> > > > > >> > >> Â Â Â Â Âif (unlikely(!page_evictable(page, NULL))) {
> > >> > > > > >> > >> Â Â Â Â Â Â Â Â Âputback_lru_page(page);
> > >> > > > > >> > >> Â Â Â Â Â Â Â Â Âcontinue;
> > >> > > > > >> > >> Â Â Â Â Â}
> > >> > > > > >> > >>
> > >> > > > > >> > >> Maybe we want to minimize the amount of code under the lru lock or
> > >> > > > > >> > >> avoid duplicate logic in the isolate_page functions.
> > >> > > > > >> > >
> > >> > > > > >> > > I guess the quick test means to avoid the expensive page_referenced()
> > >> > > > > >> > > call that follows it. But that should be mostly one shot cost - the
> > >> > > > > >> > > unevictable pages are unlikely to cycle in active/inactive list again
> > >> > > > > >> > > and again.
> > >> > > > > >> >
> > >> > > > > >> > Please read what putback_lru_page does.
> > >> > > > > >> >
> > >> > > > > >> > It moves the page onto the unevictable list, so that
> > >> > > > > >> > it will not end up in this scan again.
> > >> > > > > >>
> > >> > > > > >> Yes it does. I said 'mostly' because there is a small hole that an
> > >> > > > > >> unevictable page may be scanned but still not moved to unevictable
> > >> > > > > >> list: when a page is mapped in two places, the first pte has the
> > >> > > > > >> referenced bit set, the _second_ VMA has VM_LOCKED bit set, then
> > >> > > > > >> page_referenced() will return 1 and shrink_page_list() will move it
> > >> > > > > >> into active list instead of unevictable list. Shall we fix this rare
> > >> > > > > >> case?
> > >> > > > >
> > >> > > > > I think it's not a big deal.
> > >> > > >
> > >> > > > Maybe, otherwise I should bring up this issue long time before :)
> > >> > > >
> > >> > > > > As you mentioned, it's rare case so there would be few pages in active
> > >> > > > > list instead of unevictable list.
> > >> > > >
> > >> > > > Yes.
> > >> > > >
> > >> > > > > When next time to scan comes, we can try to move the pages into
> > >> > > > > unevictable list, again.
> > >> > > >
> > >> > > > Will PG_mlocked be set by then? Otherwise the situation is not likely
> > >> > > > to change and the VM_LOCKED pages may circulate in active/inactive
> > >> > > > list for countless times.
> > >> > >
> > >> > > PG_mlocked is not important in that case.
> > >> > > Important thing is VM_LOCKED vma.
> > >> > > I think below annotaion can help you to understand my point. :)
> > >> >
> > >> > Hmm, it looks like pages under VM_LOCKED vma is guaranteed to have
> > >> > PG_mlocked set, and so will be caught by page_evictable(). Is it?
> > >>
> > >> No. I am sorry for making my point not clear.
> > >> I meant following as.
> > >> When the next time to scan,
> > >>
> > >> shrink_page_list
> > > Â->
> > > Â Â Â Â Â Â Â Âreferenced = page_referenced(page, 1,
> > > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âsc->mem_cgroup, &vm_flags);
> > > Â Â Â Â Â Â Â Â/* In active use or really unfreeable? ÂActivate it. */
> > > Â Â Â Â Â Â Â Âif (sc->order <= PAGE_ALLOC_COSTLY_ORDER &&
> > > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âreferenced && page_mapping_inuse(page))
> > > Â Â Â Â Â Â Â Â Â Â Â Âgoto activate_locked;
> > >
> > >> -> try_to_unmap
> > > Â Â ~~~~~~~~~~~~ this line won't be reached if page is found to be
> > > Â Â referenced in the above lines?
> > 
> > Indeed! In fact, I was worry about that.
> > It looks after live lock problem.
> > But I think  it's very small race window so  there isn't any report until now.
> > Let's Cced Lee.
> > 
> > If we have to fix it, how about this ?
> > This version  has small overhead than yours since
> > there is less shrink_page_list call than page_referenced.
> 
> Yeah, it looks better. However I still wonder if (VM_LOCKED && !PG_mlocked)
> is possible and somehow persistent. Does anyone have the answer? Thanks!

hehe, that's bug. you spotted very good thing IMHO ;)
I posted fixed patch. can you see it?



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