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

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Message-ID: <557E911F.5040602@jp.fujitsu.com>
Date:	Mon, 15 Jun 2015 17:47:27 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"Luck, Tony" <tony.luck@...el.com>, Xishi Qiu <qiuxishi@...wei.com>
CC:	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 2015/06/11 5:40, Luck, Tony wrote:
>> I guess, mirrored memory should be allocated if !__GFP_HIGHMEM or !__GFP_MOVABLE
>
> HIGHMEM shouldn't matter - partial memory mirror only makes any sense on X86_64 systems ... 32-bit kernels
> don't even boot on systems with 64GB, and the minimum rational configuration for a machine that supports
> mirror is 128GB (4 cpu sockets * 2 memory controller per socket * 4 channels per controller * 4GB DIMM ...
> leaving any channels empty likely leaves you short of memory bandwidth for these high core count processors).
>
> MOVABLE is mostly the opposite of MIRROR - we never want to fill a kernel allocation from a MOVABLE page. I
> want all kernel allocations to be from MIRROR.
>

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)

Thanks,
-Kame


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