Message-ID: <50AB646E.7040009@jp.fujitsu.com>
Date: Tue, 20 Nov 2012 20:07:26 +0900
From: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Tang Chen <tangchen@cn.fujitsu.com>, <wency@cn.fujitsu.com>,
        <linfeng@cn.fujitsu.com>, <rob@landley.net>, <laijs@cn.fujitsu.com>,
        <jiang.liu@huawei.com>, <kosaki.motohiro@jp.fujitsu.com>,
        <minchan.kim@gmail.com>, <mgorman@suse.de>, <rientjes@google.com>,
        <yinghai@kernel.org>, <rusty@rustcorp.com.au>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        <linux-doc@vger.kernel.org>
Subject: Re: [PATCH 0/5] Add movablecore_map boot option.
References: <1353335246-9127-1-git-send-email-tangchen@cn.fujitsu.com> <20121119125325.ed1abba0.akpm@linux-foundation.org>
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2012/11/20 5:53, Andrew Morton wrote:
> On Mon, 19 Nov 2012 22:27:21 +0800
> Tang Chen <tangchen@cn.fujitsu.com> wrote:
>
>> This patchset provide a boot option for user to specify ZONE_MOVABLE memory
>> map for each node in the system.
>>
>> movablecore_map=nn[KMG]@ss[KMG]
>>
>> This option make sure memory range from ss to ss+nn is movable memory.
>> 1) If the range is involved in a single node, then from ss to the end of
>>     the node will be ZONE_MOVABLE.
>> 2) If the range covers two or more nodes, then from ss to the end of
>>     the node will be ZONE_MOVABLE, and all the other nodes will only
>>     have ZONE_MOVABLE.
>> 3) If no range is in the node, then the node will have no ZONE_MOVABLE
>>     unless kernelcore or movablecore is specified.
>> 4) This option could be specified at most MAX_NUMNODES times.
>> 5) If kernelcore or movablecore is also specified, movablecore_map will have
>>     higher priority to be satisfied.
>> 6) This option has no conflict with memmap option.
>
> This doesn't describe the problem which the patchset solves.  I can
> kinda see where it's coming from, but it would be nice to have it all
> spelled out, please.
>

> - What is wrong with the kernel as it stands?

If we hot remove a memroy, the memory cannot have kernel memory,
because Linux cannot migrate kernel memory currently. Therefore,
we have to guarantee that the hot removed memory has only movable
memoroy.

Linux has two boot options, kernelcore= and movablecore=, for
creating movable memory. These boot options can specify the amount
of memory use as kernel or movable memory. Using them, we can
create ZONE_MOVABLE which has only movable memory.

But it does not fulfill a requirement of memory hot remove, because
even if we specify the boot options, movable memory is distributed
in each node evenly. So when we want to hot remove memory which
memory range is 0x80000000-0c0000000, we have no way to specify
the memory as movable memory.

So we proposed a new feature which specifies memory range to use as
movable memory.

> - What are the possible ways of solving this?

I thought 2 ways to specify movable memory.
  1. use firmware information
  2. use boot option

1. use firmware information
   According to ACPI spec 5.0, SRAT table has memory affinity structure
   and the structure has Hot Pluggable Filed. See "5.2.16.2 Memory
   Affinity Structure". If we use the information, we might be able to
   specify movable memory by firmware. For example, if Hot Pluggable
   Filed is enabled, Linux sets the memory as movable memory.

2. use boot option
   This is our proposal. New boot option can specify memory range to use
   as movable memory.

> - Describe the chosen way, explain why it is superior to alternatives

We chose second way, because if we use first way, users cannot change
memory range to use as movable memory easily. We think if we create
movable memory, performance regression may occur by NUMA. In this case,
user can turn off the feature easily if we prepare the boot option.
And if we prepare the boot optino, the user can select which memory
to use as movable memory easily.

Thanks,
Yasuaki Ishimatsu

>
> The amount of manual system configuration in this proposal looks quite
> high.  Adding kernel boot parameters really is a last resort.  Why was
> it unavoidable here?
>


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