linux-kernel - Re: [PATCH v9 2/5] ksm: count all zero pages placed by KSM

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Date:   Wed, 24 May 2023 09:23:12 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Yang Yang <yang.yang29@....com.cn>, akpm@...ux-foundation.org
Cc:     imbrenda@...ux.ibm.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
Subject: Re: [PATCH v9 2/5] ksm: count all zero pages placed by KSM

On 24.05.23 07:57, Yang Yang wrote:
> From: xu xin <xu.xin16@....com.cn>
> 
> As pages_sharing and pages_shared don't include the number of zero pages
> merged by KSM, 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. In the early days of use_zero_pages,
> zero-pages was unable to get unshared by the ways like MADV_UNMERGEABLE so
> it's hard to count how many times one of those zeropages was then unmerged.
> 
> But now, unsharing KSM-placed zero page accurately has been achieved, so we
> can easily count both how many times a page full of zeroes was merged with
> zero-page and how many times one of those pages was then unmerged. and so,
> it helps to estimate memory demands when each and every shared page could
> get unshared.
> 
> So we add ksm_zero_pages under /sys/kernel/mm/ksm/ to show the number
> of all zero pages placed by KSM. Meanwhile, we update the Documentation.
> 
> Signed-off-by: xu xin <xu.xin16@....com.cn>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Cc: Xuexin Jiang <jiang.xuexin@....com.cn>
> Reviewed-by: Xiaokai Ran <ran.xiaokai@....com.cn>
> Reviewed-by: Yang Yang <yang.yang29@....com.cn>
> ---
>   Documentation/admin-guide/mm/ksm.rst |  7 +++++++
>   include/linux/ksm.h                  | 12 ++++++++++++
>   mm/khugepaged.c                      |  2 ++
>   mm/ksm.c                             | 12 ++++++++++++
>   mm/memory.c                          |  5 ++++-
>   5 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
> index 7626392fe82c..6cc919dbfd55 100644
> --- a/Documentation/admin-guide/mm/ksm.rst
> +++ b/Documentation/admin-guide/mm/ksm.rst
> @@ -173,6 +173,13 @@ stable_node_chains
>           the number of KSM pages that hit the ``max_page_sharing`` limit
>   stable_node_dups
>           number of duplicated KSM pages
> +ksm_zero_pages
> +        how many zero pages that are still mapped into processes were mapped by
> +        KSM when deduplicating.
> +
> +When ``use_zero_pages`` is/was enabled, the sum of ``pages_sharing`` +
> +``ksm_zero_pages`` represents the actual number of pages saved by KSM.
> +if ``use_zero_pages`` has never been enabled, ``ksm_zero_pages`` is 0.
>   
>   A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
>   sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index 4fd5f4a50bac..f2d98c53cfec 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -33,6 +33,14 @@ void __ksm_exit(struct mm_struct *mm);
>    */
>   #define is_ksm_zero_pte(pte)	(is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte))
>   
> +extern unsigned long ksm_zero_pages;
> +
> +static inline void ksm_notify_unmap_zero_page(pte_t pte)
> +{
> +	if (is_ksm_zero_pte(pte))
> +		ksm_zero_pages--;
> +}
> +
>   static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
>   {
>   	int ret;
> @@ -103,6 +111,10 @@ static inline void ksm_exit(struct mm_struct *mm)
>   
>   #define is_ksm_zero_pte(pte)	0
>   
> +static inline void ksm_notify_unmap_zero_page(pte_t pte)
> +{
> +}
> +

Having proposed that name ... I realize that we call this function 
whenever there might be a zeropage mapped (when we have !page after 
vm_normal_page()) -- but it could also not be the zeropage.

Not really able to come up with a better name :)

ksm_notify_maybe_unmap_zero_page ?

ksm_maybe_unmap_zero_page ?


Maybe someone else reading along has a better idea. In any case, the 
logic itself LGTM

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb