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Date:   Thu, 3 Dec 2020 01:03:47 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] mm/page_isolation: do not isolate the max order page

On 12/2/20 1:21 PM, Muchun Song wrote:
> The max order page has no buddy page and never merge to other order.
> So isolating and then freeing it is pointless.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/page_isolation.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index a254e1f370a3..bddf788f45bf 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>  	 */
>  	if (PageBuddy(page)) {
>  		order = buddy_order(page);
> -		if (order >= pageblock_order) {
> +		if (order >= pageblock_order && order < MAX_ORDER - 1) {
>  			pfn = page_to_pfn(page);
>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>  			buddy = page + (buddy_pfn - pfn);

Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
!pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
might access an invalid buddy. So this might be actually a bug fix and not just
optimization, just the bug hasn't been observed in practice.

> 

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