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

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Message-ID: <4a3daba6-18f9-d252-697c-197f65578c44@redhat.com>
Date:   Thu, 29 Sep 2022 11:21:44 +0200
From:   David Hildenbrand <david@...hat.com>
To:     xu.xin.sc@...il.com, akpm@...ux-foundation.org,
        imbrenda@...ux.vnet.ibm.com
Cc:     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 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.

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

-- 
Thanks,

David / dhildenb