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

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Date:	Thu, 13 May 2010 11:19:59 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	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

On 05/13/2010 11:05 AM, KOSAKI Motohiro wrote:
>>>> 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().

Sorry, I don't see what's the problem with that. I can see only that
alloc_pages_exact is superfluous there as kmalloc "won't fail" earlier.

>>>> 	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.

Indeed. I didn't mean that as the users should change that. They should
only if there is some weird hardware with weird drivers.

>> 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?

This is possible, but the question is why to make the code more complex?

> Anyway, alloc_fdmem() also don't works as author expected.

Pardon my ignorance, why? (There are more similar users:
init_section_page_cgroup, sys_add_key, ext4_fill_flex_info and many others.)

-- 
js
suse labs
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