linux-kernel - Re: [PATCH] mm/page_reporting: the "page" must not be the list head

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Message-ID: <20200818084121.GA33710@L-31X9LVDL-1304.local>
Date:   Tue, 18 Aug 2020 16:41:21 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_reporting: the "page" must not be the list head

On Tue, Aug 18, 2020 at 09:23:12AM +0200, David Hildenbrand wrote:
>On 18.08.20 05:05, Wei Yang wrote:
>> On Mon, Aug 17, 2020 at 07:07:04PM +0200, David Hildenbrand wrote:
>>> On 17.08.20 18:05, Alexander Duyck wrote:
>>>>
>>>>
>>>> On 8/17/2020 2:35 AM, David Hildenbrand wrote:
>>>>> On 17.08.20 10:48, Wei Yang wrote:
>>>>>> If "page" is the list head, list_for_each_entry_safe() would stop
>>>>>> iteration.
>>>>>>
>>>>>> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
>>>>>> ---
>>>>>>   mm/page_reporting.c | 2 +-
>>>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
>>>>>> index 3bbd471cfc81..aaaa3605123d 100644
>>>>>> --- a/mm/page_reporting.c
>>>>>> +++ b/mm/page_reporting.c
>>>>>> @@ -178,7 +178,7 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,
>>>>>>   		 * the new head of the free list before we release the
>>>>>>   		 * zone lock.
>>>>>>   		 */
>>>>>> -		if (&page->lru != list && !list_is_first(&page->lru, list))
>>>>>> +		if (!list_is_first(&page->lru, list))
>>>>>>   			list_rotate_to_front(&page->lru, list);
>>>>>>   
>>>>>>   		/* release lock before waiting on report processing */
>>>>>>
>>>>>
>>>>> Is this a fix or a cleanup? If it's a fix, can this be reproduced easily
>>>>> and what ere the effects?
>>>>>
>>>>
>>>> This should be a clean-up. Since the &page->lru != list will always be true.
>>>>
>>>
>>> Makes sense, maybe we can make that a little bit clearer in the patch
>>> description.
>>>
>> 
>> Ok, do you have some suggestion on the description?
>> 
>>    A clean-up for commit xxx?
>> 
>> I would appreciate your suggestion :-)
>> 
>
>I'd go with something like
>
>"
>mm/page_reporting: drop stale list head check in page_reporting_cycle
>
>list_for_each_entry_safe() guarantees that we will never stumble over
>the list head; "&page->lru != list" will always evaluate to true. Let's
>simplify.
>"
>

Looks really better than mine. Thanks a lot.

>to stress that this is a pure simplifcation.
>
>Reviewed-by: David Hildenbrand <david@...hat.com>
>
>>>> If I recall at some point the that was a check for &next->lru != list 
>>>> but I think I pulled out an additional conditional check somewhere so 
>>>> that we just go through the start of the loop again and iterate over 
>>>> reported pages until we are guaranteed to have a non-reported page to 
>>>> rotate to the top of the list with the general idea being that we wanted 
>>>> the allocator to pull non-reported pages before reported pages.
>>>
>>> -- 
>>> Thanks,
>>>
>>> David / dhildenb
>> 
>
>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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