linux-kernel - Re: [PATCH 1/7] mm: introduce simple_malloc()/simple

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Message-ID: <4920F682.3010601@linux.vnet.ibm.com>
Date:	Mon, 17 Nov 2008 10:13:46 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Dave Airlie <airlied@...il.com>
CC:	Arjan van de Ven <arjan@...radead.org>,
	David Miller <davem@...emloft.net>, laijs@...fujitsu.com,
	akpm@...ux-foundation.org, menage@...gle.com,
	kamezawa.hiroyu@...fujitsu.com, jens.axboe@...cle.com,
	jack@...e.cz, jes@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] mm: introduce simple_malloc()/simple_free()

Dave Airlie wrote:
> On Mon, Nov 17, 2008 at 4:57 AM, Arjan van de Ven <arjan@...radead.org> wrote:
>> On Sun, 16 Nov 2008 00:19:26 -0800 (PST)
>> David Miller <davem@...emloft.net> wrote:
>>
>>> From: Arjan van de Ven <arjan@...radead.org>
>>> Date: Sat, 15 Nov 2008 20:52:29 -0800
>>>
>>>> On Sun, 16 Nov 2008 12:33:15 +0800
>>>> Lai Jiangshan <laijs@...fujitsu.com> wrote:
>>>>
>>>>> some subsystem needs vmalloc() when required memory is large.
>>>>> but current kernel has not APIs for this requirement.
>>>>> this patch introduces simple_malloc() and simple_free().
>>>> I kinda really don't like this approach. vmalloc() (and especially,
>>>> vfree()) is a really expensive operation, and vmalloc()'d memory is
>>>> also slower (due to tlb pressure). Realistically, people should try
>>>> hard to use small datastructure instead....
>>> This is happening in many places, already, for good reason.
>>>
>>> There are lots of places where we can't (core hash tables, etc.)  and
>>> we want NUMA spreading and reliable allocation, and thus vmalloc it
>>> is.
>> vmalloc() isn't 100% evil; for truely long term stuff it's sometimes a
>> quite reasonable solution.
>>
>> There are some issues with it still: the vmalloc() space is shared
>> with ioremap, modules and others and it's not all that big on 32 bit; on
>> x86 you could well end up with only 64Mb total (after taking out the
>> various ioremap's etc).
>>
>> Yes there's places where it's then totally fine to dip into this space
>> at boot/init time. You mention a few very good users.
>> (There's still the tlb miss cost on use but on modern cpus a tlb miss
>> is actually quite cheap)
>>
>> But this doesn't make vmalloc() the magic bullet that solves the "oh
>> Linux can't allocate large chunks of memory" problem. Specifically in
>> driver space for things that get ported from other OSes.
> 
> So we keep the duplicated code? or we just audit new callers.... I
> think this patch
> makes it easier to spot new callers doing something stupid. As davem
> said we duplicate
> this code all over the place, so for that reason along a simple
> wrapper makes things a lot
> easier, and also possibly a lot easier to change in the future to a
> new non-sucky API.
> 
> So I'm all for it maybe with a non simple name.

Yes, tracking vmalloc() spillage all over the place is harder than using one
abstraction and fixing that appropriately if needed (specially for 32 bit systems).

-- 
	Balbir
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