linux-kernel - Re: [PATCH] ksm: delay the check of splitting compound pages

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Date:   Tue, 14 Nov 2023 14:02:22 +0100
From:   David Hildenbrand <david@...hat.com>
To:     yang.yang29@....com.cn, akpm@...ux-foundation.org
Cc:     imbrenda@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, ran.xiaokai@....com.cn, xu.xin.sc@...il.com,
        xu.xin16@....com.cn, jiang.xuexin@....com.cn,
        wang.yong12@....com.cn
Subject: Re: [PATCH] ksm: delay the check of splitting compound pages

On 14.11.23 13:36, yang.yang29@....com.cn wrote:
> From: xu xin <xu.xin16@....com.cn>
> 
> Background
> ==========
> When trying to merge two pages, it may fail because the two pages
> belongs to the same compound page and split_huge_page fails due to
> the incorrect reference to the page. To solve the problem, the commit
> 77da2ba0648a4 ("mm/ksm: fix interaction with THP") tries to split the
> compound page after try_to_merge_two_pages() fails and put_page in
> that case. However it is too early to calculate of the variable 'split' which
> indicates whether the two pages belongs to the same compound page.
> 
> What to do
> ==========
> If try_to_merge_two_pages() succeeds, there is no need to check whether
> to splitting compound pages. So we delay the check of splitting compound
> pages until try_to_merge_two_pages() fails, which can improve the
> processing efficiency of cmp_and_merge_page() a little.
> 
> Signed-off-by: xu xin <xu.xin16@....com.cn>
> Reviewed-by: Yang Yang <yang.yang29@....com.cn>

Can we please add a unit test to ksm_functional_tests.c so we actually 
get it right this time?

-- 
Cheers,

David / dhildenb