linux-kernel - Re: [RFC PATCH 10/12] mm: add the buddy system interface

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Date:	Tue, 16 Jun 2015 09:31:42 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"Luck, Tony" <tony.luck@...el.com>
CC:	Xishi Qiu <qiuxishi@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"nao.horiguchi@...il.com" <nao.horiguchi@...il.com>,
	Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>, Xiexiuqi <xiexiuqi@...wei.com>,
	Hanjun Guo <guohanjun@...wei.com>,
	Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 10/12] mm: add the buddy system interface

On 2015/06/16 2:20, Luck, Tony wrote:
> On Mon, Jun 15, 2015 at 05:47:27PM +0900, Kamezawa Hiroyuki wrote:
>> So, there are 3 ideas.
>>
>>   (1) kernel only from MIRROR / user only from MOVABLE (Tony)
>>   (2) kernel only from MIRROR / user from MOVABLE + MIRROR(ASAP)  (AKPM suggested)
>>       This makes use of the fact MOVABLE memory is reclaimable but Tony pointed out
>>       the memory reclaim can be critical for GFP_ATOMIC.
>>   (3) kernel only from MIRROR / user from MOVABLE, special user from MIRROR (Xishi)
>>
>> 2 Implementation ideas.
>>    - creating ZONE
>>    - creating new alloation attribute
>>
>> I don't convince whether we need some new structure in mm. Isn't it good to use
>> ZONE_MOVABLE for not-mirrored memory ?
>> Then, disable fallback from ZONE_MOVABLE -> ZONE_NORMAL for (1) and (3)
>
> We might need to rename it ... right now the memory hotplug
> people use ZONE_MOVABLE to indicate regions of physical memory
> that can be removed from the system.  I'm wondering whether
> people will want systems that have both removable and mirrored
> areas?  Then we have four attribute combinations:
>
> mirror=no  removable=no  - prefer to use for user, could use for kernel if we run out of mirror
> mirror=no  removable=yes - can only be used for user (kernel allocation makes it not-removable)
> mirror=yes removable=no  - use for kernel, possibly for special users if we define some interface
> mirror=yes removable=yes - must not use for kernel ... would have to give to user ... seems like a bad idea to configure a system this way
>

Thank you for clarification. I see "mirror=no, removable=no" case may require a new name.

IMHO, the value of Address-Based-Memory-Mirror is that users can protect their system's
important functions without using full-memory mirror. So, I feel thinking
"mirror=no, removable=no" just makes our discussion/implemenation complex without real
user value.

Shouldn't we start with just thiking 2 cases of
  mirror=no  removable=yes
  mirror=yes removable=no
?

And then, if the naming is problem, alias name can be added.

Thanks,
-Kame






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