linux-kernel - Re: bootmem allocator

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Date:	Tue, 08 Apr 2008 23:55:45 -0700
From:	Mike Travis <travis@....com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...e.hu>, Cyrill Gorcunov <gorcunov@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: bootmem allocator

Andi Kleen wrote:
> On Tue, Apr 08, 2008 at 11:00:28PM -0700, Mike Travis wrote:
>> Ingo Molnar wrote:
>>> * Andi Kleen <andi@...stfloor.org> wrote:
>>>
>>>>> hm, bootmem allocator is supposed to clear memory. We have a couple 
>>>>> of places that rely on that.
>>>> I was actually considering to change that for the GB pages hugetlbfs 
>>>> patchkit, because memset for 1G is a little slow and not needed (will 
>>>> be cleared later anyways) and it might be a problem for very large 
>>>> systems with a lot of such pages at boot.
>>> changing the default behavior of bootmem alloc to be non-clearing is a 
>>> really bad idea that will only cause unrobustness. The proper approach 
>>> is to add an _opt-in_ API that does not clear memory 
>>> (bootmem_alloc_dontclear() or whatever), available to callers that know 
>>> it for sure that they dont need the clearing.
>> Yes, changing the default of bootmem_alloc is a bad idea.  I just changed
>> a bunch of static arrays to bootmem alloc's and it was pointed out early
>> that not only does bootmem_alloc clear memory, but also panics if it's not
>> available.
> 
> There are more and more bootmem calls that don't want the panic actually.
> That is why _nopanic was invented (and gets more and more variants)
> At some point the default could be even switched.
> 
> I think the right way would be to survey the callers (there are not
> that many) and then come up with a sane single API that caters to the
> majority of them by default and passes flags for the special cases.
> 
> -Andi

Hi Andi,

I really don't care(*), but there's lot's of code that expects a certain
behavior.  Either all the source calls have to be modified en masse (and you
well know that's difficult given the _zillion_ source trees), or you have to
introduce the new API transparently.  That means leaving a backdoor for old
calls:

     #define bootmem_alloc_low(...) \
	 __new_bootmem_alloc(..., FLAGS_FOR_OLD_BOOTMEM_ALLOC_LOW);

Then I think you're free to optimize away... d;-) [happy face with a baseball cap]

Cheers,
Mike

(*) As long as I don't have to debug problems as a result of the change...! ;-)
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