linux-kernel - Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from one page into another

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Date:	Wed, 12 Nov 2008 11:19:31 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Izik Eidus <ieidus@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm@...r.kernel.org, chrisw@...hat.com, avi@...hat.com,
	izike@...ranet.com
Subject: Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from
 one page into another

On Tue, 11 Nov 2008 23:24:21 +0100
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Tue, Nov 11, 2008 at 03:31:18PM -0600, Christoph Lameter wrote:
> > > ksm need the pte inside the vma to point from anonymous page into filebacked
> > > page
> > > can migrate.c do it without changes?
> > 
> > So change anonymous to filebacked page?
> >
> > Currently page migration assumes that the page will continue to be part
> > of the existing file or anon vma.
> > 
> > What you want sounds like assigning a swap pte to an anonymous page? That
> > way a anon page gains membership in a file backed mapping.
> 
> KSM needs to convert anonymous pages to PageKSM, which means a page
> owned by ksm.c and only known by ksm.c. The Linux VM will free this
> page in munmap but that's about it, all we do is to match the number
> of anon-ptes pointing to the page with the page_count. So besides
> freeing the page when the last user exit()s or cows it, the VM will do
> nothing about it. Initially. Later it can swap it in a nonlinear way.
> 
Can I make a question ? (I'm working for memory cgroup.)

Now, we do charge to anonymous page when
  - charge(+1) when it's mapped firstly (mapcount 0->1)
  - uncharge(-1) it's fully unmapped (mapcount 1->0) vir page_remove_rmap().

My quesion is
 - PageKSM pages are not necessary to be tracked by memory cgroup ?
 - Can we know that "the page is just replaced and we don't necessary to do
   charge/uncharge".
 - annonymous page from KSM is worth to be tracked by memory cgroup ?
   (IOW, it's on LRU and can be swapped-out ?)

Thanks,
-Kame


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