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SubjectRe: bootmem allocator
Andi Kleen wrote:
> On Tue, Apr 08, 2008 at 11:00:28PM -0700, Mike Travis wrote:
>> Ingo Molnar wrote:
>>> * Andi Kleen <andi@firstfloor.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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