linux-kernel - Re: [PATCH v2 1/2] memblock: add assertion for zero allocation alignment

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Date:	Fri, 22 Feb 2013 02:17:53 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] memblock: add assertion for zero allocation alignment

On Friday 22 February 2013 02:01 AM, Yinghai Lu wrote:
> On Thu, Feb 21, 2013 at 12:10 PM, Vineet Gupta
> <Vineet.Gupta1@...opsys.com> wrote:
>> This came to light when calling memblock allocator from arc port (for
>> copying flattended DT). If a "0" alignment is passed, the allocator
>> round_up() call incorrectly rounds up the size to 0.
>>
>> round_up(num, alignto) => ((num - 1) | (alignto -1)) + 1
>>
>> While the obvious allocation failure causes kernel to panic, it is
>> better to warn the caller to fix the code.
>>
>> Tejun suggested that instead of BUG_ON(!align) - which might be
>> ineffective due to pending console init and such, it is better to
>> WARN_ON, and continue the boot with a reasonable default align.
>>
>> Caller passing @size need not be handled similarly as the subsequent
>> panic will indicate that anyhow.
>>
>> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Tejun Heo <tj@...nel.org>
>> Cc: Yinghai Lu <yinghai@...nel.org>
>> Cc: Wanpeng Li <liwanp@...ux.vnet.ibm.com>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> ---
>>  mm/memblock.c |    3 +++
>>  1 files changed, 3 insertions(+), 0 deletions(-)
>>
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index 1bcd9b9..f3804bd 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -824,6 +824,9 @@ static phys_addr_t __init memblock_alloc_base_nid(phys_addr_t size,
>>         /* align @size to avoid excessive fragmentation on reserved array */
>>         size = round_up(size, align);
>>
>> +       if (WARN_ON(!align))
>> +               align = __alignof__(long long);
>> +
> the checking should be put before round_up?

Oops my bad.

Interestingly however, I did test this exact patch on ARC before sending out -
passing @align = 0 to make it hit the WARN. It prints the warning, and uses
@size=0, @align=8 for memblock_find_in_range_node() and successfully allocates
memory as opposed to failure for @size=0, @align=0 scenario. This is kind of weird.

Anyhow I'll send the updated patch to fix the gotcha !

Thx,
-Vineet
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