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

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Date:   Thu, 24 Mar 2022 09:54:53 +0100
From:   David Hildenbrand <david@...hat.com>
To:     CGEL <cgel.zte@...il.com>
Cc:     akpm@...ux-foundation.org, 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 24.03.22 02:36, CGEL wrote:
> On Wed, Mar 23, 2022 at 07:43:04PM +0100, David Hildenbrand wrote:
>> 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.
>>
> I think we need failed count. Because ksm cow oom is a little different
> from general OOM. User may wonder I am not allocing new memory, why it
> cause OOM? And OOM may have big impact for some kind of tasks, so we
> better let user know that, do we?

The log will be flooded by messages from the OOM handler, how will one
counter regarding KSM help? I really don't think this is of any use.

-- 
Thanks,

David / dhildenb