linux-kernel - Re: page-flags behavior on compound pages: a worry

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Date:	Wed, 12 Aug 2015 16:47:53 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hugh Dickins <hughd@...gle.com>,
	David Rientjes <rientjes@...gle.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: page-flags behavior on compound pages: a worry

On 08/12/2015 04:35 PM, Kirill A. Shutemov wrote:
> On Thu, Aug 06, 2015 at 12:24:22PM -0700, Hugh Dickins wrote:
>>> IIUC, the only potentially problematic callsites left are physical memory
>>> scanners. This code requires audit. I'll do that.
>>
>> Please.
>
> I haven't finished the exercise yet. But here's an issue I believe present
> in current *Linus* tree:
>
>  From e78eec7d7a8c4cba8b5952a997973f7741e704f4 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Date: Wed, 12 Aug 2015 17:09:16 +0300
> Subject: [PATCH] mm: fix potential race in isolate_migratepages_block()
>
> Hugh has pointed that compound_head() call can be unsafe in some context.
> There's one example:
>
> 	CPU0					CPU1
>
> isolate_migratepages_block()
>    page_count()
>      compound_head()
>        !!PageTail() == true
> 					put_page()
> 					  tail->first_page = NULL
>        head = tail->first_page
> 					alloc_pages(__GFP_COMP)
> 					   prep_compound_page()
> 					     tail->first_page = head
> 					     __SetPageTail(p);
>        !!PageTail() == true
>      <head == NULL dereferencing>
>
> The race is pure theoretical. I don't it's possible to trigger it in
> practice. But who knows.

It's even less probable thanks to the fact that before this check we 
determined it's a PageLRU (and thus !PageTail).

>
> This can be fixed by avoiding compound_head() in unsafe context.

This is OK because if page becomes tail and we read zero page count, 
it's not fatal.

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Fixes: 119d6d59dc ("mm, compaction: avoid isolating pinned pages")

Potentially stable 3.15+ if theoretical races qualify. They don't per 
stable rules, but we seem to be bending that a lot anyway.

> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>

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

> ---
>   mm/compaction.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 86f04e556f96..bec727b700d3 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -787,7 +787,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   		 * admittedly racy check.
>   		 */
>   		if (!page_mapping(page) &&
> -		    page_count(page) > page_mapcount(page))
> +		    atomic_read(&page->_count) > page_mapcount(page))
>   			continue;
>
>   		/* If we already hold the lock, we can skip some rechecking */
>

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