linux-kernel - Re: [PATCH v3 0/5] ksm: support tracking KSM-placed zero-pages

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Date:   Mon, 17 Oct 2022 16:55:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     xu.xin.sc@...il.com
Cc:     ran.xiaokai@....com.cn, yang.yang29@....com.cn,
        jiang.xuexin@....com.cn, imbrenda@...ux.ibm.com, david@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        xu xin <xu.xin16@....com.cn>
Subject: Re: [PATCH v3 0/5] ksm: support tracking KSM-placed zero-pages

On Tue, 11 Oct 2022 02:20:06 +0000 xu.xin.sc@...il.com wrote:

> From: xu xin <xu.xin16@....com.cn>
> 
> use_zero_pages is good, not just because of cache colouring as described
> in doc, but also because use_zero_pages can accelerate merging empty pages
> when there are plenty of empty pages (full of zeros) as the time of
> page-by-page comparisons (unstable_tree_search_insert) is saved.
> 
> But there is something to improve, that is, when enabling use_zero_pages,
> all empty pages will be merged with kernel zero pages instead of with each
> other as use_zero_pages is disabled, and then these zero-pages are no longer
> managed and monitor by KSM, which leads to two issues at least:

Sorry, but I'm struggling to understand what real value this patchset
offers.

> 1) 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); see the link:
>    https://lore.kernel.org/lkml/4a3daba6-18f9-d252-697c-197f65578c44@redhat.com/

Is that causing users any real-world problem?  If not, just change the
documentation?

> 2) we cannot know how many pages are zero pages placed by KSM when
>    enabling use_zero_pages, which leads to KSM not being transparent
>    with all actual merged pages by KSM.

Why is this a problem?

A full description of the real-world end-user operational benefits of
these changes would help, please.