linux-kernel - Re: [PATCH] perf/ring_buffer: Fix invalid page order

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Date:   Tue, 15 Nov 2016 09:48:14 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Takao Indoh <indou.takao@...fujitsu.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf/ring_buffer: Fix invalid page order


* Takao Indoh <indou.takao@...fujitsu.com> wrote:

> In rb_alloc_aux_page(), a page order is set to MAX_ORDER when order is
> greater than MAX_ORDER, but page order should be less than MAX_ORDER,
> therefore alloc_pages_node fails at least once. This patch fixes page
> order so that it can be always less than MAX_ORDER.
> 
> Signed-off-by: Takao Indoh <indou.takao@...fujitsu.com>
> ---
>  kernel/events/ring_buffer.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 257fa46..3f76fdd 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -502,8 +502,8 @@ static struct page *rb_alloc_aux_page(int node, int order)
>  {
>  	struct page *page;
>  
> -	if (order > MAX_ORDER)
> -		order = MAX_ORDER;
> +	if (order >= MAX_ORDER)
> +		order = MAX_ORDER - 1;
>  
>  	do {
>  		page = alloc_pages_node(node, PERF_AUX_GFP, order);

I'm wondering under what circumstances this allocation failure was seen in 
practice - why did others not hit this?

Thanks,

	Ingo