linux-kernel - Re: [PATCH] mm: page_alloc: avoid false page outside zone error info

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Date:   Wed, 5 Jul 2023 09:43:58 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Matthew Wilcox <willy@...radead.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: avoid false page outside zone error info

On 2023/7/5 0:38, Andrew Morton wrote:
> On Tue, 4 Jul 2023 20:36:00 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:
> 
>> On 2023/7/4 20:13, Matthew Wilcox wrote:
>>> On Tue, Jul 04, 2023 at 07:18:23PM +0800, Miaohe Lin wrote:
>>>> @@ -470,6 +470,8 @@ static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
>>>>  		sp = zone->spanned_pages;
>>>>  		if (!zone_spans_pfn(zone, pfn))
>>>>  			ret = 1;
>>>> +		else
>>>> +			ret = 0;
>>>
>>> Surely 'ret = zone_spans_pfn(zone, pfn);' ?
>>
>> Do you mean 'ret = !zone_spans_pfn(zone, pfn);'?  This format looks fine to me.
>>
>>>
>>> Also, did you spot this by inspection or do you have a test-case or bug
>>> report?  Should this have a Fixes: tag?
>>
>> This is from code inspection. The race window should be really small thus hard to trigger
>> in real world. And yes, it seems Fixes tag is a really ancient commit:
>>
>> 	Fixes: bdc8cb984576 ("[PATCH] memory hotplug locking: zone span seqlock")
>>
> 
> Thanks.  I updated the changelog:
> 
> : If pfn is outside zone boundaries in the first round, ret will be set to
> : 1.  But if pfn is changed to inside the zone boundaries in zone span
> : seqretry path, ret is still set to 1 leading to false page outside zone
> : error info.
> : 
> : This is from code inspection.  The race window should be really small thus
> : hard to trigger in real world.
> : 
> : Link: https://lkml.kernel.org/r/20230704111823.940331-1-linmiaohe@huawei.com
> : Fixes: bdc8cb984576 ("[PATCH] memory hotplug locking: zone span seqlock")
> 
> and I made the change suggested by Matthew:
> 
> --- a/mm/page_alloc.c~mm-page_alloc-avoid-false-page-outside-zone-error-info-fix
> +++ a/mm/page_alloc.c
> @@ -468,10 +468,7 @@ static int page_outside_zone_boundaries(
>  		seq = zone_span_seqbegin(zone);
>  		start_pfn = zone->zone_start_pfn;
>  		sp = zone->spanned_pages;
> -		if (!zone_spans_pfn(zone, pfn))
> -			ret = 1;
> -		else
> -			ret = 0;
> +		ret = !zone_spans_pfn(zone, pfn);
>  	} while (zone_span_seqretry(zone, seq));

These changes look good to me. Thanks for doing this.