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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Changli Gao <xiaosuo@gmail.com>
Subject: Re: [RFC] mm: generic adaptive large memory allocation APIs
Cc: kosaki.motohiro@jp.fujitsu.com, akpm@linux-foundation.org,
       Eric Dumazet <eric.dumazet@gmail.com>, Jiri Slaby <jslaby@suse.cz>,
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       Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
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Date: Thu, 13 May 2010 13:45:35 +0900 (JST)
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Hi

> void *kvmalloc(size_t size)
> {
> 	void *ptr;
> 
> 	if (size < PAGE_SIZE)
> 		return kmalloc(PAGE_SIZE, GFP_KERNEL);
> 	ptr = alloc_pages_exact(size, GFP_KERNEL | __GFP_NOWARN);

low order GFP_KERNEL allocation never fail. then, this doesn't works
as you expected.

> 	if (ptr != NULL)
> 		return ptr;
> 
> 	return vmalloc(size);

On x86, vmalloc area is only 128MB address space. it is very rare 
resource than physical ram. vmalloc fallback is not good idea.