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

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Message-Id: <20100907174743.2efa34bd.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 7 Sep 2010 17:47:43 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.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, 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

 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)

I'll start from adding ALLOC_FIXED.

Thanks,
-Kame

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