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

[KOSAKI Motohiro]

Hi

> Greetings,
> 
> Jeff Dike found that many KVM pages are being refaulted in 2.6.29:
> 
> "Lots of pages between discarded due to memory pressure only to be
> faulted back in soon after. These pages are nearly all stack pages.
> This is not consistent - sometimes there are relatively few such pages
> and they are spread out between processes."

I suprise this result really.

  - Why this issue happened only on kvm?
  - Why shrink_inactive_list() can't find pte young bit?
    Is this really unused stack?

> 
> The refaults can be drastically reduced by the following patch, which
> respects the referenced bit of all anonymous pages (including the KVM
> pages).
> 
> However it risks reintroducing the problem addressed by commit 7e9cd4842
> (fix reclaim scalability problem by ignoring the referenced bit,
> mainly the pte young bit). I wonder if there are better solutions?
> 
> Thanks,
> Fengguang
> 
> ---
>  mm/vmscan.c |   10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> --- linux.orig/mm/vmscan.c
> +++ linux/mm/vmscan.c
> @@ -1288,12 +1288,12 @@ static void shrink_active_list(unsigned 
>  			 * Identify referenced, file-backed active pages and
>  			 * give them one more trip around the active list. So
>  			 * that executable code get better chances to stay in
> -			 * memory under moderate memory pressure.  Anon pages
> -			 * are not likely to be evicted by use-once streaming
> -			 * IO, plus JVM can create lots of anon VM_EXEC pages,
> -			 * so we ignore them here.
> +			 * memory under moderate memory pressure.
> +			 *
> +			 * Also protect anon pages: swapping could be costly,
> +			 * and KVM guest's referenced bit is helpful.
>  			 */
> -			if ((vm_flags & VM_EXEC) && !PageAnon(page)) {
> +			if ((vm_flags & VM_EXEC) || PageAnon(page)) {
>  				list_add(&page->lru, &l_active);
>  				continue;
>  			}



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