linux-kernel - Re: [RFC][PATCH] big continuous memory allocator v2

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Date:	Tue, 7 Sep 2010 23:51:26 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mel Gorman <mel@....ul.ie>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC][PATCH] big continuous memory allocator v2

On Tue, Sep 07, 2010 at 05:47:43PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 7 Sep 2010 01:37:27 -0700
> Minchan Kim <minchan.kim@...il.com> wrote:
> 
> > Nice cleanup.
> > There are some comments in below.
> > 
> > On Mon, Sep 6, 2010 at 7:45 PM, KAMEZAWA Hiroyuki
> > <kamezawa.hiroyu@...fujitsu.com> wrote:
> > >
> > > This is a page allcoator based on memory migration/hotplug code.
> > > passed some small tests, and maybe easier to read than previous one.
> > >
> > > ==
> > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > >
> > > This patch as a memory allocator for contiguous memory larger than MAX_ORDER.
> > >
> > > ??alloc_contig_pages(hint, size, node);
> > 
> > I have thought this patch is to be good for dumb device drivers which
> > want big contiguous
> > memory. So if some device driver want big memory and they can tolerate
> > latency or fail,
> > this is good solution, I think.
> > And some device driver can't tolerate fail, they have to use MOVABLE zone.
> > 
> > For it, I hope we have a option like ALLOC_FIXED(like MAP_FIXED).
> > That's because embedded people wanted to aware BANK of memory.
> > So if they get free page which they don't want, it can be pointless.
> > 
> Okay.
> 
> 
> > In addition, I hope it can support CROSS_ZONE migration mode.
> > Most of small system can't support swap system. So if we can't migrate
> > anon pages into other zones, external fragment problem still happens.
> > 
> Now, this code migrates pages to somewhere, including crossing zone, node etc..
> (because it just use GFP_HIGHUSER_MOVABLE)
> 
> > I think reclaim(ex, discard file-backed pages) can become one option to prevent
> > the problem. But it's more cost so we can support it by calling mode.
> > (But it could be trivial since caller should know this function is very cost)
> > 
> 
> > ex) alloc_contig_pages(hint, size, node, ALLOC_FIXED|ALLOC_RECLAIM);
> > 
> 
> This migration's page allocation code will cause memory reclaim and
> kswapd wakeup if memory is in short. But hmm, there are no codes as

Yes. But it's useless. That's because it's not a zone/node we want to reclaim.
The zone we want to reclaim is not alloc failed zone but the zone which include 
alloc_contig_pages's hint address. 

> 
>  reclaim_memory_within(start, end).
> 
> But I guess if there are LRU pages within the range which cannot be migrated,
> they can't be dropped. In another consideration, 
> 
>   shrink_slab_within(start, end)
> will be able to make success-rate better. (and this is good for memory hotplug, too)

And it can help normal external memory fragement, too. 

> 
> I'll start from adding ALLOC_FIXED.

I am looking forward to seeing your next version. :)
Thanks, Kame. 
-- 
Kind regards,
Minchan Kim
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