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

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Date:   Thu, 29 Sep 2022 12:36:30 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     xu.xin.sc@...il.com, akpm@...ux-foundation.org,
        imbrenda@...ux.vnet.ibm.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, xu xin <xu.xin16@....com.cn>
Subject: Re: [PATCH 0/3] ksm: fix incorrect count of merged pages when
 enabling use_zero_pages

On Thu, 29 Sep 2022 11:21:44 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 29.09.22 04:52, 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 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).
> > 
> > Please see the subsequent patches for details.  
> 
> Just raising the topic here because it's related to the KSM usage of the 
> shared zero-page:
> 
> MADV_UNMERGEABLE and other ways to trigger unsharing will *not* unshare 
> the shared zeropage as placed by KSM (which is against the 
> MADV_UNMERGEABLE documentation at least). It will only unshare actual 
> KSM pages. We might not want want to blindly unshare all shared 
> zeropages in applicable VMAs ... using a dedicated shared zero (KSM) 
> page -- instead of the generic zero page --  might be one way to handle 
> this cleaner.

I don't understand why do you need this.

first of all, one zero page would not be enough (depending on the
architecture, e.g. on s390x you need many). the whole point of zero
page merging is that one zero page is not enough.

second, once a page is merged with a zero page, it's not really handled
by KSM anymore. if you have a big allocation, of which you only touch a
few pages, would the rest be considered "merged"? no, it's just zero
pages, right?
this is the same, except that we take present pages with zeroes in it
and we discard them and map them to zero pages. it's kinda like if we
had never touched them.

> 
> Would that also fix some of the issues you describe above?
>