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Message-ID: <d87bfcdd-cbaa-4485-a63b-6a524681fa08@redhat.com>
Date:   Wed, 15 Nov 2023 11:49:24 +0100
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Subject: Re: [PATCH] ksm: delay the check of splitting compound pages
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To:     xu <xu.xin.sc@gmail.com>
Cc:     akpm@linux-foundation.org, imbrenda@linux.ibm.com,
        jiang.xuexin@zte.com.cn, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, ran.xiaokai@zte.com.cn, wang.yong12@zte.com.cn,
        xu.xin16@zte.com.cn, yang.yang29@zte.com.cn
References: <b4a11840-0118-44af-9397-30b5bcdd6552@redhat.com>
 <20231115031129.1970581-1-xu.xin16@zte.com.cn>
From:   David Hildenbrand <david@redhat.com>
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In-Reply-To: <20231115031129.1970581-1-xu.xin16@zte.com.cn>
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On 15.11.23 04:11, xu wrote:
>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>> index 7efcc68ccc6e..c952fe5d9e43 100644
>>> --- a/mm/ksm.c
>>> +++ b/mm/ksm.c
>>> @@ -2229,24 +2229,10 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
>>>    	tree_rmap_item =
>>>    		unstable_tree_search_insert(rmap_item, page, &tree_page);
>>>    	if (tree_rmap_item) {
>>> -		bool split;
>>> -
>>>    		kpage = try_to_merge_two_pages(rmap_item, page,
>>>    						tree_rmap_item, tree_page);
>>> -		/*
>>> -		 * If both pages we tried to merge belong to the same compound
>>> -		 * page, then we actually ended up increasing the reference
>>> -		 * count of the same compound page twice, and split_huge_page
>>> -		 * failed.
>>> -		 * Here we set a flag if that happened, and we use it later to
>>> -		 * try split_huge_page again. Since we call put_page right
>>> -		 * afterwards, the reference count will be correct and
>>> -		 * split_huge_page should succeed.
>>> -		 */
>>
>> I'm curious, why can't we detect that ahead of time and keep only a
>> single reference? Why do we need the backup code? Anything I am missing?
> 
> I don't know the original reason, better ask Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>.
> Maybe because doing detection that ahead of time will break several funtions' semantic,
> such as try_to_merge_two_pages(), try_to_merge_with_ksm_page() and try_to_merge_one_page()
> 
> Adding the backup code don't change the old code and fixing the old problem, it's good.

It's absolutely counter-intuitive to check for something that cannot 
possibly work after the effects. This better has a good reason to make 
that code more complicated.
-- 
Cheers,

David / dhildenb