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    SubjectRe: [RFC] respect the referenced bit of KVM guest pages?
    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 -
    > >> > >>  Is there a good reason for this to be in shrink_active_list()
    > >> > >> as opposed to __isolate_lru_page?
    > >> > >>
    > >> > >>          if (unlikely(!page_evictable(page, NULL))) {
    > >> > >>                  putback_lru_page(page);
    > >> > >>                  continue;
    > >> > >>          }
    > >> > >>
    > >> > >> 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        2009-08-16 19:11:13.000000000 +0800
    > > +++ linux/mm/rmap.c     2009-08-16 19:22:46.000000000 +0800
    > > @@ -358,6 +358,7 @@ static int page_referenced_one(struct pa
    > >         */
    > >        if (vma->vm_flags & VM_LOCKED) {
    > >                *mapcount = 1;  /* break early from loop */
    > > +               *vm_flags |= VM_LOCKED;
    > >                goto out_unmap;
    > >        }
    > >
    > > @@ -482,6 +483,8 @@ static int page_referenced_file(struct p
    > >        }
    > >
    > >        spin_unlock(&mapping->i_mmap_lock);
    > > +       if (*vm_flags & VM_LOCKED)
    > > +               referenced = 0;
    > >        return referenced;
    > >  }
    > >
    > >
    > > --
    > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
    > > the body to majordomo@kvack.org.  For more info on Linux MM,
    > > see: http://www.linux-mm.org/ .
    > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
    > >
    >
    >
    >
    > --
    > Kind regards,
    > Minchan Kim
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