linux-kernel - Re: [PATCH v2] mm/page_isolation: do not isolate the max order page

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Date:   Mon, 7 Dec 2020 14:42:08 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v2] mm/page_isolation: do not isolate the max order page

On Fri, Dec 04, 2020 at 12:22:37AM +0800, 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. And 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 is also a bug fix.
> 
> Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

> ---
> Changes in v2:
>  - Add Fixes tag in the commit log.
> 
>  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);
> -- 
> 2.11.0
> 
> 

-- 
Oscar Salvador
SUSE L3