linux-kernel - Re: [PATCH v2] mm/page_isolation: fix potential missing call to unset_migratetype

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Message-ID: <YT9AS6I1Th14mCxh@dhcp22.suse.cz>
Date:   Mon, 13 Sep 2021 14:12:59 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, david@...hat.com, vbabka@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_isolation: fix potential missing call to
 unset_migratetype_isolate()

On Mon 13-09-21 19:51:25, 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 simplify the code further.

I like the clean up part but is this a real problem that requires CC
stable? Have you ever seen this to be a real problem? It looks like
something based on reading the code.

> Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> Cc: <stable@...r.kernel.org>
> ---
> v1->v2:
>   Simplify the code further per David Hildenbrand.
> ---
>  mm/page_isolation.c | 20 +++-----------------
>  1 file changed, 3 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index a95c2c6562d0..f93cc63d8fa1 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -183,7 +183,6 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>  			     unsigned migratetype, int flags)
>  {
>  	unsigned long pfn;
> -	unsigned long undo_pfn;
>  	struct page *page;
>  
>  	BUG_ON(!IS_ALIGNED(start_pfn, pageblock_nr_pages));
> @@ -193,25 +192,12 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>  	     pfn < end_pfn;
>  	     pfn += pageblock_nr_pages) {
>  		page = __first_valid_page(pfn, pageblock_nr_pages);
> -		if (page) {
> -			if (set_migratetype_isolate(page, migratetype, flags)) {
> -				undo_pfn = pfn;
> -				goto undo;
> -			}
> +		if (page && set_migratetype_isolate(page, migratetype, flags)) {
> +			undo_isolate_page_range(start_pfn, pfn, migratetype);
> +			return -EBUSY;
>  		}
>  	}
>  	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);
> -	}
> -
> -	return -EBUSY;
>  }
>  
>  /*
> -- 
> 2.23.0

-- 
Michal Hocko
SUSE Labs