linux-kernel - Re: [RFC] mm: memblock: change default cnt for regions from 1 to 0

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Date:	Mon, 27 Oct 2014 14:38:27 +0000
From:	Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com>
To:	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	Tang Chen <tangchen@...fujitsu.com>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC] mm: memblock: change default cnt for regions from 1 to
 0


On 27/10/14 14:17, Tejun Heo wrote:
> Hello,
> 
> On Thu, Oct 23, 2014 at 12:18:40PM -0700, Andrew Morton wrote:
>> On Thu, 23 Oct 2014 17:56:53 +0100 Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com> wrote:
>>
>>> The default region counts are set to 1 with a comment saying empty
>>> dummy entry.
>>>
>>> If this is a dummy entry, should this be changed to 0?
> 
> My memory is hazy now but I'm pretty sure there's a bunch of stuff
> assuming that the array is never empty.
> 
>>> We have faced this in mips/kernel/setup.c arch_mem_init.
>>>
>>> cma uses memblock. But even with cma disabled.
>>> The for_each_memblock(reserved, reg) goes inside the loop.
>>> Even without any reserved regions.
> 
> Does that matter?  It's a zero-length reservation.
> 
>>> Traced it to the following, when the macro
>>> for_each_memblock(memblock_type, region) is used.
>>>
>>> It expands to add the cnt variable.
>>>
>>> for (region = memblock.memblock_type.regions; 		\
>>> 	region < (memblock.memblock_type.regions + memblock.memblock_type.cnt); \
>>> 	region++)
>>>
>>> In the corner case, that there are no reserved regions.
>>> Due to the default 1 value of cnt.
>>> The loop under for_each_memblock still runs once.
>>>
>>> Even when there is no reserved region.
>>>
>>> Is this by design? or unintentional?
> 
> It's by design.
> 
>>> It might be that this loop runs an extra time every instance out there?
> 
> The first actual entry replaces the dummy one and the last removal
> makes the entry dummy again, so the dummy one exists iff that's the
> only entry.  I don't recall the exact details right now but the choice
> was an intentional one.
> 
> Thanks.
> 

Thank-you for clarifying.

Regards
ZubairLK
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