linux-kernel - Re: [PATCH v2 0/5] ksm: support tracking KSM-placed zero-pages

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Date:   Mon, 10 Oct 2022 12:08:34 +0000
From:   xu xin <xu.xin.sc@...il.com>
To:     imbrenda@...ux.ibm.com
Cc:     akpm@...ux-foundation.org, david@...hat.com,
        jiang.xuexin@....com.cn, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, ran.xiaokai@....com.cn, xu.xin.sc@...il.com,
        xu.xin16@....com.cn, yang.yang29@....com.cn
Subject: Re: [PATCH v2 0/5] ksm: support tracking KSM-placed zero-pages

Hello, Thanks for your reply.

>
>why are you trying so hard to fix something that is not broken?

Actually, it breaks the definition of unmerge, though it's usually not a big
problem.
>
>can't you just avoid using use_zero_pages?

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 comparision
(unstable_tree_search_insert) is saved.

>
>why is it so important to know how many zero pages have been merged?
>and why do you want to unmerge them?

Zero pages may be the most common merged pages in actual environment(not only VM but
also including other application like containers). Sometimes customers (app developers)
are very interested in how many non-zero-pages are actually merged in their apps.

>
>the important thing is that the sysadmin knows how much memory would be
>needed to do the unmerge, and that information is already there.
>

I think it's about chicken-and-egg problem.

Anyway, thanks for your reply.