linux-kernel - Re: [PATCH 1/1] mm/cma: fix alloc_contig

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Message-ID: <20171122120002.GA27270@cmpxchg.org>
Date:   Wed, 22 Nov 2017 07:00:02 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Nazarewicz <mina86@...a86.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/cma: fix alloc_contig_range ret code/potential
 leak

On Mon, Nov 20, 2017 at 11:39:30AM -0800, Mike Kravetz wrote:
> If the call __alloc_contig_migrate_range() in alloc_contig_range
> returns -EBUSY, processing continues so that test_pages_isolated()
> is called where there is a tracepoint to identify the busy pages.
> However, it is possible for busy pages to become available between
> the calls to these two routines.  In this case, the range of pages
> may be allocated.   Unfortunately, the original return code (ret
> == -EBUSY) is still set and returned to the caller.  Therefore,
> the caller believes the pages were not allocated and they are leaked.
> 
> Update the return code with the value from test_pages_isolated().
> 
> Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

Wow, good catch.

> ---
>  mm/page_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77e4d3c5c57b..3605ca82fd29 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7632,10 +7632,10 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	}
>  
>  	/* Make sure the range is really isolated. */
> -	if (test_pages_isolated(outer_start, end, false)) {
> +	ret = test_pages_isolated(outer_start, end, false);
> +	if (ret) {
>  		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
>  			__func__, outer_start, end);
> -		ret = -EBUSY;
>  		goto done;

Essentially, an -EBUSY from __alloc_contig_migrate_range() doesn't
mean anything, and we return 0 if the rest of the operations succeed.

Since we never plan on returning that particular -EBUSY, would it be
more robust to reset it right then and there, rather than letting it
run on in ret for more than a screenful?

It would also be good to note in that fall-through comment that the
pages becoming free on their own is a distinct possibility.

As Michal points out, this is really subtle. It makes sense to make it
as explicit as possible.