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

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Date:   Tue, 4 Jul 2023 09:38:19 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
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 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));
 
 	if (ret)
_