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

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Date:   Tue, 15 Nov 2016 18:11:05 +0900
From:   Takao Indoh <indou.takao@...fujitsu.com>
To:     <mingo@...nel.org>
CC:     <peterz@...radead.org>, <mingo@...hat.com>, <acme@...nel.org>,
        <alexander.shishkin@...ux.intel.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf/ring_buffer: Fix invalid page order

On 2016/11/15 17:48, Ingo Molnar wrote:
>
> * 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?

I found this when I ran perf with -m,2048. I think in the most cases
users use default buffer size hence they does not notice.

Thanks,
Takao Indoh