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Message-Id: <20221017165541.6e2d3cebdc1ba13861ea4b2b@linux-foundation.org>
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.
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