Re: [S+Q 09/16] [percpu] make allocpercpu usable during early boot

[Pekka Enberg <penberg@cs.helsinki.fi>]

Christoph Lameter wrote:
> On Thu, 1 Jul 2010, Pekka Enberg wrote:
> 
>>> On Mon, 28 Jun 2010, Pekka Enberg wrote:
>>>>> +               return kzalloc(size, GFP_KERNEL & gfp_allowed_mask);
>>>>>        else {
>>>>>                void *ptr = vmalloc(size);
>>>>>                if (ptr)
>>>> This looks wrong to me. All slab allocators should do gfp_allowed_mask
>>>> magic under the hood. Maybe it's triggering kmalloc_large() path that
>>>> needs the masking too?
>> On Tue, Jun 29, 2010 at 6:45 PM, Christoph Lameter
>> <cl@linux-foundation.org> wrote:
>>> They do gfp_allowed_mask magic. But the checks at function entry of the
>>> slabs do not mask the masks so we get false positives without this. All my
>>> protest against the checks doing it this IMHO broken way were ignored.
>> Which checks are those? Are they in SLUB proper or are they introduced
>> in one of the SLEB patches? We definitely don't want to expose
>> gfp_allowed_mask here.
> 
> Argh. The reason for the trouble here is because I moved the
> masking of the gfp flags out of the hot path.
> 
> The masking of the bits adds to the cache footprint of the hotpaths now in
> all slab allocators. Gosh. Why is there constant contamination of the hot
> paths with the stuff?

We should definitely move gfp_allowed_mask masking out of fast-paths. I 
think the ideal solution is to only do it deep in the page allocator.

> We only need this masking in the hot path if the debugging hooks need it.
> Otherwise its fine to defer this to the slow paths.
> 
> So how do I get that in there? Add "& gfp_allowed_mask" to the gfp mask
> passed to the debugging hooks?
> 
> Or add a debug_hooks_alloc() function and make it empty if no debugging
> functions are enabled?

I think it's best to have separate debugging hooks that are empty if 
debugging is disabled, yes.

			Pekka

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