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

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Message-Id: <20220929111315.284133-1-xu.xin16@zte.com.cn>
Date:   Thu, 29 Sep 2022 11:13:15 +0000
From:   xu xin <xu.xin.sc@...il.com>
To:     david@...hat.com
Cc:     akpm@...ux-foundation.org, imbrenda@...ux.vnet.ibm.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xu.xin.sc@...il.com, xu.xin16@....com.cn
Subject: Reply:[PATCH 0/3] ksm: fix incorrect count of merged pages when enabling use_zero_pages

>> 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?

Glad to see your reply. I think it depends.

The way you said solves the issue you post, but maybe not help to solve the issue
I post.

The key lies in whether appending zeropage's rmap_items to stable tree. If
appending their rmap_items to the stable tree, the issue I pointed can be fixed but
that will degrade the performance of use_zero_pages.  If not appending their rmap_items
to the stable tree, we have to choose this patches set (but I found some bugs now, later
I will send v2 to fix it).