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 13:11:12 +0200
From:	Izik Eidus <izik@...ranet.com>
To:	Avi Kivity <avi@...hat.com>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	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, izike@...ranet.com
Subject: Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from
 one page into another

Avi Kivity wrote:
> KAMEZAWA Hiroyuki wrote:
>> 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 ?
When we reaplacing page using page_replace() we have:
oldpage - > anonymous page that is going to be replaced by newpage
newpage -> kernel allocated page (KsmPage)
so about oldpage we are calling page_remove_rmap() that will notify cgroup
and about newpage it wont be count inside cgroup beacuse it is file rmap 
page
(we are calling to page_add_file_rmap), so right now PageKSM wont ever 
be tracked by cgroup.

>>  - Can we know that "the page is just replaced and we don't necessary 
>> to do
>>    charge/uncharge".

The caller of page_replace does know it, the only problem is that 
page_remove_rmap()
automaticly change the cgroup for anonymous pages,
if we want it not to change the cgroup, we can:
increase the cgroup count before page_remove (but in that case what 
happen if we reach to the limit???)
give parameter to page_remove_rmap() that we dont want the cgroup to be 
changed.

>>  - annonymous page from KSM is worth to be tracked by memory cgroup ?
>>    (IOW, it's on LRU and can be swapped-out ?)

KSM have no anonymous pages (it share anonymous pages into KsmPAGE -> 
kernel allocated page without mapping)
so it isnt in LRU and it cannt be swapped, only when KsmPAGEs will be 
break by do_wp_page() the duplication will be able to swap.

>>   
>
> My feeling is that shared pages should be accounted as if they were 
> not shared; that is, a share page should be accounted for each process 
> that shares it.  Perhaps sharing within a cgroup should be counted as 
> 1 page for all the ptes pointing to it.
>
>
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