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

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Date:   Thu, 16 Nov 2023 18:39:01 +0100
From:   David Hildenbrand <david@...hat.com>
To:     xu <xu.xin.sc@...il.com>
Cc:     akpm@...ux-foundation.org, imbrenda@...ux.ibm.com,
        jiang.xuexin@....com.cn, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, ran.xiaokai@....com.cn, wang.yong12@....com.cn,
        xu.xin16@....com.cn, yang.yang29@....com.cn
Subject: Re: [PATCH] ksm: delay the check of splitting compound pages

On 16.11.23 13:17, xu wrote:
>>>>> @@ -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?
> 
> Do you mean like this?

Let me see if the refcounting here is sane:

(a) The caller has a reference on "page" that it will put just after the
     cmp_and_merge_page() call.
(b) unstable_tree_search_insert() obtained a reference to the
     "tree_page" using get_mergeable_page()->follow_page(). We will put
     that reference.

So indeed, if both references are to the same folio, *we* have two 
references to the folio and can safely drop one of both.

> 
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2229,23 +2229,21 @@ 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;
> +               bool SameCompound;
> +               /*
> +                * If they belongs to the same compound page, its' reference
> +                * get twice, so need to put_page once to avoid that
> +                * split_huge_page fails in try_to_merge_two_pages().
> +                */
> +               if (SameCompound = Is_SameCompound(page, tree_page))
> +                       put_page(tree_page);
>   

bool same_folio = page_folio(page) == page_folio(tree_page);

/*
  * If both pages belong to the same folio, we are holding two references
  * to the same large folio: splitting the folio in
  * try_to_merge_one_page() will fail for that reason. So let's just drop
  * one reference early.  Note that this is only possible if tree_page is
  * not a KSM page yet.
  */
if (same_folio)
	put_page(tree_page);

[we could use more folio operations here, but lets KIS]

-- 
Cheers,

David / dhildenb