linux-kernel - Re: [PATCH] mm/page_isolation: fix potential missing call to unset_migratetype

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Date:   Mon, 6 Sep 2021 11:27:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     mhocko@...e.com, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_isolation: fix potential missing call to
 unset_migratetype_isolate()

On 04.09.21 11:20, Miaohe Lin wrote:
> In start_isolate_page_range() undo path, pfn_to_online_page() just checks
> the first pfn in a pageblock while __first_valid_page() will traverse the
> pageblock until the first online pfn is found. So we may miss the call to
> unset_migratetype_isolate() in undo path and pages will remain isolated
> unexpectedly. Fix this by calling undo_isolate_page_range() and this will
> also help to remove some duplicated codes.
> 
> Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")

While that is true, we shouldn't ever trigger, neither via cma, 
virtio-mem nor memory offlining, because essentially all operate on 
MAX_ORDER -1 -aligned ranges without memory holes.

> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>   mm/page_isolation.c | 9 +--------
>   1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 471e3a13b541..9bb562d5d194 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -202,14 +202,7 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>   	}
>   	return 0;
>   undo:
> -	for (pfn = start_pfn;
> -	     pfn < undo_pfn;
> -	     pfn += pageblock_nr_pages) {
> -		struct page *page = pfn_to_online_page(pfn);
> -		if (!page)
> -			continue;
> -		unset_migratetype_isolate(page, migratetype);
> -	}
> +	undo_isolate_page_range(start_pfn, undo_pfn, migratetype);
>   

It'd be even cleaner to drop the label completely and call it from the 
single callsite. We can even avoid undo_pfn ...

if (page && set_migratetype_isolate(page, migratetype, flags)) {
	undo_isolate_page_range(start_pfn, pfn, migratetype);
	return -EBUSY;
}

If pfn == start_pfn, undo_isolate_page_range() will simply do nothing.

>   	return -EBUSY;
>   }
> 

-- 
Thanks,

David / dhildenb