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

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Aug 19, 2009 at 09:38:05PM +0800, Minchan Kim wrote:
> On Wed, Aug 19, 2009 at 10:24 PM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> > On Wed, Aug 19, 2009 at 08:25:56PM +0800, Minchan Kim wrote:
> >> On Wed, Aug 19, 2009 at 9:10 PM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> >> > On Wed, Aug 19, 2009 at 08:05:19PM +0800, KOSAKI Motohiro wrote:
> >> >> >> page_referenced_file?
> >> >> >> I think we should change page_referenced().
> >> >> >
> >> >> > Yeah, good catch.
> >> >> >
> >> >> >>
> >> >> >> Instead, How about this?
> >> >> >> ==============================================
> >> >> >>
> >> >> >> Subject: [PATCH] mm: stop circulating of referenced mlocked pages
> >> >> >>
> >> >> >> Currently, mlock() systemcall doesn't gurantee to mark the page PG_Mlocked
> >> >> >
> >> >> >                                                    mark PG_mlocked
> >> >> >
> >> >> >> because some race prevent page grabbing.
> >> >> >> In that case, instead vmscan move the page to unevictable lru.
> >> >> >>
> >> >> >> However, Recently Wu Fengguang pointed out current vmscan logic isn't so
> >> >> >> efficient.
> >> >> >> mlocked page can move circulatly active and inactive list because
> >> >> >> vmscan check the page is referenced _before_ cull mlocked page.
> >> >> >>
> >> >> >> Plus, vmscan should mark PG_Mlocked when cull mlocked page.
> >> >> >
> >> >> >                           PG_mlocked
> >> >> >
> >> >> >> Otherwise vm stastics show strange number.
> >> >> >>
> >> >> >> This patch does that.
> >> >> >
> >> >> > Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
> >> >>
> >> >> Thanks.
> >> >>
> >> >>
> >> >>
> >> >> >> Index: b/mm/rmap.c
> >> >> >> ===================================================================
> >> >> >> --- a/mm/rmap.c       2009-08-18 19:48:14.000000000 +0900
> >> >> >> +++ b/mm/rmap.c       2009-08-18 23:47:34.000000000 +0900
> >> >> >> @@ -362,7 +362,9 @@ static int page_referenced_one(struct pa
> >> >> >>        * unevictable list.
> >> >> >>        */
> >> >> >>       if (vma->vm_flags & VM_LOCKED) {
> >> >> >> -             *mapcount = 1;  /* break early from loop */
> >> >> >> +             *mapcount = 1;          /* break early from loop */
> >> >> >> +             *vm_flags |= VM_LOCKED; /* for prevent to move active list */
> >> >> >
> >> >> >> +             try_set_page_mlocked(vma, page);
> >> >> >
> >> >> > That call is not absolutely necessary?
> >> >>
> >> >> Why? I haven't catch your point.
> >> >
> >> > Because we'll eventually hit another try_set_page_mlocked() when
> >> > trying to unmap the page. Ie. duplicated with another call you added
> >> > in this patch.
> >>
> >> Yes. we don't have to call it and we can make patch simple.
> >> I already sent patch on yesterday.
> >>
> >> http://marc.info/?l=linux-mm&m=125059325722370&w=2
> >>
> >> I think It's more simple than KOSAKI's idea.
> >> Is any problem in my patch ?
> >
> > No, IMHO your patch is simple and good, while KOSAKI's is more
> > complete :)
> >
> > - the try_set_page_mlocked() rename is suitable
> > - the call to try_set_page_mlocked() is necessary on try_to_unmap()
> 
> We don't need try_set_page_mlocked call in try_to_unmap.
> That's because try_to_unmap_xxx will call try_to_mlock_page if the
> page is included in any VM_LOCKED vma. Eventually, It can move
> unevictable list.

Yes, indeed!

> > - the "if (VM_LOCKED) referenced = 0" in page_referenced() could
> >  cover both active/inactive vmscan
> 
> ASAP we set PG_mlocked in page, we can save unnecessary vmscan cost from
> active list to inactive list. But I think it's rare case so that there
> would be few pages.
> So I think that will be not big overhead.

The active list case can be persistent, when the mlocked (but without
PG_mlocked) page is executable and referenced by 2+ processes. But I
admit that executable pages are relatively rare.

> As I know, Rescue by vmscan page losing the isolation race was the
> Lee's design.
> But as you pointed out, it have a bug that vmscan can't rescue the
> page due to reach try_to_unmap.
> 
> So I think this approach is proper. :)

Now you decide :)

Thanks,
Fengguang

> > I did like your proposed
> >
> >                if (sc->order <= PAGE_ALLOC_COSTLY_ORDER &&
> > -                                       referenced && page_mapping_inuse(page))
> > +                                       referenced && page_mapping_inuse(page)
> > +                                       && !(vm_flags & VM_LOCKED))
> >                        goto activate_locked;
> >
> > which looks more intuitive and less confusing.
> >
> > Thanks,
> > Fengguang
> >
> 
> 
> 
> -- 
> Kind regards,
> Minchan Kim