linux-kernel - Re: [RFC] mm: generic adaptive large memory allocation APIs

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Date:	Thu, 13 May 2010 18:05:25 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	kosaki.motohiro@...fujitsu.com, Changli Gao <xiaosuo@...il.com>,
	akpm@...ux-foundation.org, Eric Dumazet <eric.dumazet@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [RFC] mm: generic adaptive large memory allocation APIs

Hi

> > 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.
> 
> Hi, I suppose you mean the kmalloc allocation -- so kmalloc should fail
> iff alloc_pages_exact (unless somebody frees a heap of memory indeed)?

I mean, if size of alloc_pages_exact() argument is less than 8 pages,
alloc_pages_exact() never fail. see __alloc_pages_slowpath().

> 
> >> 	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.
> 
> These functions are a replacement for explicit
> if (!(x = kmalloc()))
>    x = vmalloc();
> ...
> if (is_vmalloc(x))
>   vfree(x);
> else
>   kfree(x);
> in the code (like fdtable does this).
> 
> The 128M limit on x86_32 for vmalloc is configurable so if drivers in
> sum need more on some specific hardware, it can be increased on the
> command line (I had to do this on one machine in the past).

Right, but 99% end user don't do this. I don't think this is effective advise.


> Anyway as this is a replacement for explicit tests, it shouldn't change
> the behaviour in any way. Obviously when a user doesn't need virtually
> contiguous space, he shouldn't use this interface at all.

Why can't we make fdtable virtually contiguous free?
Anyway, alloc_fdmem() also don't works as author expected.



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