From: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
To: Tejun Heo <tj@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Tang Chen <tangchen@cn.fujitsu.com>,
Yinghai Lu <yinghai@kernel.org>
Subject: Re: [RFC] mm: memblock: change default cnt for regions from 1 to 0
Date: Mon, 27 Oct 2014 14:38:27 +0000 [thread overview]
Message-ID: <544E58E3.9040100@imgtec.com> (raw)
In-Reply-To: <20141027141730.GL4436@htj.dyndns.org>
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@imgtec.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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prev parent reply other threads:[~2014-10-27 14:40 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-23 16:56 Zubair Lutfullah Kakakhel
2014-10-23 19:18 ` Andrew Morton
2014-10-27 14:17 ` Tejun Heo
2014-10-27 14:38 ` Zubair Lutfullah Kakakhel [this message]
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