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

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Date:   Tue, 18 Aug 2020 09:23:12 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     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 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.
"

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