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

[Wu Fengguang]

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?
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
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