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

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

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@intel.com> 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 -
> >> > >> A Is there a good reason for this to be in shrink_active_list()
> >> > >> as opposed to __isolate_lru_page?
> >> > >>
> >> > >> A  A  A  A  A if (unlikely(!page_evictable(page, NULL))) {
> >> > >> A  A  A  A  A  A  A  A  A putback_lru_page(page);
> >> > >> A  A  A  A  A  A  A  A  A continue;
> >> > >> A  A  A  A  A }
> >> > >>
> >> > >> 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.

> As I know about mlock pages, we already had some races condition.
> They will be rescued like above.

Thanks,
Fengguang

> >
> > How about this fix?
> >
> > ---
> > mm: stop circulating of referenced mlocked pages
> >
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> >
> > --- linux.orig/mm/rmap.c A  A  A  A 2009-08-16 19:11:13.000000000 +0800
> > +++ linux/mm/rmap.c A  A  2009-08-16 19:22:46.000000000 +0800
> > @@ -358,6 +358,7 @@ static int page_referenced_one(struct pa
> > A  A  A  A  */
> > A  A  A  A if (vma->vm_flags & VM_LOCKED) {
> > A  A  A  A  A  A  A  A *mapcount = 1; A /* break early from loop */
> > + A  A  A  A  A  A  A  *vm_flags |= VM_LOCKED;
> > A  A  A  A  A  A  A  A goto out_unmap;
> > A  A  A  A }
> >
> > @@ -482,6 +483,8 @@ static int page_referenced_file(struct p
> > A  A  A  A }
> >
> > A  A  A  A spin_unlock(&mapping->i_mmap_lock);
> > + A  A  A  if (*vm_flags & VM_LOCKED)
> > + A  A  A  A  A  A  A  referenced = 0;
> > A  A  A  A return referenced;
> > A }
> >
> >
> > --
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> >
> 
> 
> 
> -- 
> Kind regards,
> Minchan Kim

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