linux-kernel - Re: [PATCH v2] mm/vmstat: add events for ksm cow

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Date:   Wed, 23 Mar 2022 19:43:04 +0100
From:   David Hildenbrand <david@...hat.com>
To:     cgel.zte@...il.com, akpm@...ux-foundation.org
Cc:     yang.yang29@....com.cn, dave.hansen@...ux.intel.com,
        ran.xiaokai@....com.cn, yang.shi@...ux.alibaba.com,
        saravanand@...com, minchan@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xu xin <xu.xin16@....com.cn>
Subject: Re: [PATCH v2] mm/vmstat: add events for ksm cow

On 23.03.22 08:57, cgel.zte@...il.com wrote:
> From: Yang Yang <yang.yang29@....com.cn>
> 
> Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want
> to save memory, it's a tradeoff by suffering delay on ksm cow. Users can
> get to know how much memory ksm saved by reading
> /sys/kernel/mm/ksm/pages_sharing, but they don't know what's the costs
> of ksm cow, and this is important of some delay sensitive tasks.
> 
> So add ksm cow events to help users evaluate whether or how to use ksm.
> 
> Signed-off-by: Yang Yang <yang.yang29@....com.cn>
> Reviewed-by: xu xin <xu.xin16@....com.cn>
> Reviewed-by: Ran Xiaokai <ran.xiaokai@....com.cn>
> ---
> v2:
> - fix compile error when CONFIG_KSM is not set
> ---
>  include/linux/vm_event_item.h |  2 ++
>  mm/memory.c                   | 20 +++++++++++++++++---
>  mm/vmstat.c                   |  2 ++
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 16a0a4fd000b..6f32be04212f 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -131,6 +131,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		SWAP_RA_HIT,
>  #ifdef CONFIG_KSM
>  		KSM_SWPIN_COPY,
> +		KSM_COW_SUCCESS,
> +		KSM_COW_FAIL,
>  #endif
>  #endif
>  #ifdef CONFIG_X86
> diff --git a/mm/memory.c b/mm/memory.c
> index 4111f97c91a0..c24d5f04fab5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3257,6 +3257,8 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  	__releases(vmf->ptl)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
> +	vm_fault_t ret = 0;
> +	bool ksm = 0;
>  
>  	if (userfaultfd_pte_wp(vma, *vmf->pte)) {
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -3294,6 +3296,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  	 */
>  	if (PageAnon(vmf->page)) {
>  		struct page *page = vmf->page;
> +		ksm = PageKsm(page);
>  
>  		/*
>  		 * We have to verify under page lock: these early checks are
> @@ -3302,7 +3305,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  		 *
>  		 * PageKsm() doesn't necessarily raise the page refcount.
>  		 */
> -		if (PageKsm(page) || page_count(page) > 3)
> +		if (ksm || page_count(page) > 3)
>  			goto copy;
>  		if (!PageLRU(page))
>  			/*
> @@ -3316,7 +3319,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  			goto copy;
>  		if (PageSwapCache(page))
>  			try_to_free_swap(page);
> -		if (PageKsm(page) || page_count(page) != 1) {
> +		if (ksm || page_count(page) != 1) {

I think we really want to recheck here, after locking the page.
Consequently, just do a PageKsm() check below.

>  			unlock_page(page);
>  			goto copy;
>  		}
> @@ -3339,7 +3342,18 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  	get_page(vmf->page);
>  
>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
> -	return wp_page_copy(vmf);
> +	ret = wp_page_copy(vmf);
> +
> +#ifdef CONFIG_KSM
> +	if (ksm) {
> +		if (unlikely(ret & VM_FAULT_ERROR))
> +			count_vm_event(KSM_COW_FAIL);
> +		else
> +			count_vm_event(KSM_COW_SUCCESS);
> +	}
> +#endif

Do we really care if we failed or not? I mean, the failure case will
usually make your app crash either way ... due to OOM.


Doing

#ifdef CONFIG_KSM
if (PageKsm(page))
	count_vm_event(COW_KSM);
#endif

before the wp_page_copy(vmf) should be good enough, no?

Should be good enough IMHO.

-- 
Thanks,

David / dhildenb