linux-kernel - Re: Reply:[PATCH 0/3] ksm: fix incorrect count of merged pages when enabling use_zero

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Message-ID: <20220930114112.4d212bfe@p-imbrenda>
Date:   Fri, 30 Sep 2022 11:41:12 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     xu xin <xu.xin.sc@...il.com>
Cc:     david@...hat.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xu.xin16@....com.cn
Subject: Re: Reply:[PATCH 0/3] ksm: fix incorrect count of merged pages when
 enabling use_zero_pages

On Fri, 30 Sep 2022 02:00:32 +0000
xu xin <xu.xin.sc@...il.com> wrote:

> >> On 29.09.22 12:42, Claudio Imbrenda wrote:  
> >> > On Thu, 29 Sep 2022 02:52:06 +0000
> >> > xu.xin.sc@...il.com wrote:
> >> >     
> >> >> From: xu xin <xu.xin16@....com.cn>
> >> >>
> >> >> Before enabling use_zero_pages by setting /sys/kernel/mm/ksm/
> >> >> use_zero_pages to 1, pages_sharing of KSM is basically accurate. But
> >> >> after enabling use_zero_pages, all empty pages that are merged with
> >> >> kernel zero page are not counted in pages_sharing or pages_shared.    
> >> > 
> >> > that's because those pages are not shared between different processes.    
> >> 
> >> They are probably the most shared pages between processes in the kernel.   
> >
> >shared from the kernel, though, not from other processes (that's what I
> >meant)
> >  
> >> They are simply not KSM pages, that's what makes accounting tricky here.  
> >
> >exactly. and those pages get shared all the time even without KSM, so
> >why care about those now?
> >
> >does it make a difference why a page is a zero page?  
> 
> WI's necessary to show these sharing zeros pages. Because:
> 
> 1) Turning on/off use_zero_pages shouldn't make it so not transparent with the
>    sharing zero pages. When administrators enable KSM and turn on use_zero_pages,
>    if much memory increases due to zero pages sharing but they don't know the
>    reasons compared to turning off use_zero_pages, isn't it confusing?

I'm not sure I understand what you mean here

> 
> 2) If no need to let users know how many full-zero-filled pages are merged by KSM
>    due to use_zero_pages, then also no need to show pages_sharing and pages_shared

that's not true.

showing pages_sharing and pages_shared helps understand how much memory
would be needed for unsharing everything.

when pages filled with zeroes are replaced with zero-pages, those will
not get unshared. there is no way to know if those were actual
zero-pages from the beginning, or if they were pages full of zeroes
that have been replaced by KSM.

>    to users. Besides, the description of pages_sharing in Documentation is wrong and
>    MISLEADING when enabling use_zero_pages.

I guess I can fix the documentation

> 
> 3) As David supposes, it also help for estimating memory demands when each and every
>    shared page could get unshared.

the current statistics show exactly how much memory would be needed
if everything would get unshared. zero-pages would not get unshared.

> >  
> >>   
> >> >     
> >> >> That is because the rmap_items of these ksm zero pages are not
> >> >> appended to The Stable Tree of KSM.
> >> >>
> >> >> We need to add the count of empty pages to let users know how many empty
> >> >> pages are merged with kernel zero page(s).    
> >> > 
> >> > why?
> >> > 
> >> > do you need to know how many untouched zero pages a process has?
> >> > 
> >> > does it make a difference if the zero page is really untouched or if it
> >> > was touched in the past but it is now zero?    
> >> 
> >> I'd also like to understand the rationale. Is it about estimating memory 
> >> demands when each and every shared page could get unshared?
> >>   
> >