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    SubjectRe: [RFC] respect the referenced bit of KVM guest pages?
    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@intel.com> 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@intel.com> 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@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.
    > > >
    > > > 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
    ->
    referenced = page_referenced(page, 1,
    sc->mem_cgroup, &vm_flags);
    /* In active use or really unfreeable? Activate it. */
    if (sc->order <= PAGE_ALLOC_COSTLY_ORDER &&
    referenced && page_mapping_inuse(page))
    goto activate_locked;

    > -> try_to_unmap
    ~~~~~~~~~~~~ this line won't be reached if page is found to be
    referenced in the above lines?

    Thanks,
    Fengguang

    > -> try_to_unmap_xxx
    > -> if (vma->vm_flags & VM_LOCKED)
    > -> try_to_mlock_page
    > -> TestSetPageMlocked
    > -> putback_lru_page
    >
    > So at last, the page will be located in unevictable list.
    >
    > > Then I was worrying about a null problem. Sorry for the confusion!
    > >
    > > Thanks,
    > > Fengguang
    > >
    > > > ----
    > > >
    > > > /*
    > > > * called from munlock()/munmap() path with page supposedly on the LRU.
    > > > *
    > > > * Note: unlike mlock_vma_page(), we can't just clear the PageMlocked
    > > > * [in try_to_munlock()] and then attempt to isolate the page. We must
    > > > * isolate the page to keep others from messing with its unevictable
    > > > * and mlocked state while trying to munlock. However, we pre-clear the
    > > > * mlocked state anyway as we might lose the isolation race and we might
    > > > * not get another chance to clear PageMlocked. If we successfully
    > > > * isolate the page and try_to_munlock() detects other VM_LOCKED vmas
    > > > * mapping the page, it will restore the PageMlocked state, unless the page
    > > > * is mapped in a non-linear vma. So, we go ahead and SetPageMlocked(),
    > > > * perhaps redundantly.
    > > > * If we lose the isolation race, and the page is mapped by other VM_LOCKED
    > > > * vmas, we'll detect this in vmscan--via try_to_munlock() or try_to_unmap()
    > > > * either of which will restore the PageMlocked state by calling
    > > > * mlock_vma_page() above, if it can grab the vma's mmap sem.
    > > > */
    > > > static void munlock_vma_page(struct page *page)
    > > > {
    > > > ...
    > > >
    > > > --
    > > > Kind regards,
    > > > Minchan Kim
    >
    >
    > --
    > Kind regards,
    > Minchan Kim
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