Re: [PATCH 08/10] mm/slab: Allow dynamic kmalloc() minimum alignment

[Hyeonggon Yoo]

On Thu, Apr 07, 2022 at 10:35:04AM +0100, Catalin Marinas wrote:
> On Thu, Apr 07, 2022 at 06:18:16PM +0900, Hyeonggon Yoo wrote:
> > On Thu, Apr 07, 2022 at 09:50:23AM +0100, Catalin Marinas wrote:
> > > On Thu, Apr 07, 2022 at 03:46:37AM +0000, Hyeonggon Yoo wrote:
> > > > On Tue, Apr 05, 2022 at 02:57:56PM +0100, Catalin Marinas wrote:
> > > > > --- a/mm/slab_common.c
> > > > > +++ b/mm/slab_common.c
> > > > > @@ -838,9 +838,18 @@ void __init setup_kmalloc_cache_index_table(void)
> > > > >  	}
> > > > >  }
> > > > >  
> > > > > -static void __init
> > > > > +unsigned int __weak arch_kmalloc_minalign(void)
> > > > > +{
> > > > > +	return ARCH_KMALLOC_MINALIGN;
> > > > > +}
> > > > > +
> > > > 
> > > > As ARCH_KMALLOC_ALIGN and arch_kmalloc_minalign() may not be same after
> > > > patch 10, I think s/ARCH_KMALLOC_ALIGN/arch_kmalloc_minalign/g
> > > > for every user of it would be more correct?
> > > 
> > > Not if the code currently using ARCH_KMALLOC_MINALIGN needs a constant.
> > > Yes, there probably are a few places where the code can cope with a
> > > dynamic arch_kmalloc_minalign() but there are two other cases where a
> > > constant is needed:
> > > 
> > > 1. As a BUILD_BUG check because the code is storing some flags in the
> > >    bottom bits of a pointer. A smaller ARCH_KMALLOC_MINALIGN works just
> > >    fine here.
> > > 
> > > 2. As a static alignment for DMA requirements. That's where the newly
> > >    exposed ARCH_DMA_MINALIGN should be used.
> > > 
> > > Note that this series doesn't make the situation any worse than before
> > > since ARCH_DMA_MINALIGN stays at 128 bytes for arm64. Current users can
> > > evolve to use a dynamic alignment in future patches. My main aim with
> > > this series is to be able to create kmalloc-64 caches on arm64.
> > 
> > AFAIK there are bunch of drivers that directly calls kmalloc().
> 
> Well, lots of drivers call kmalloc() ;).
> 
> > It becomes tricky when e.g.) a driver allocates just 32 bytes,
> > but architecture requires it to be 128-byte aligned.
> 
> That's the current behaviour, a 32 byte allocation would return an
> object from kmalloc-128. I want to reduce this to at least kmalloc-64
> (or smaller) if the CPU/SoC allows it.

Yeah I agree the change is worth :) Thanks for the work.

> > That's why everything allocated from kmalloc() need to be aligned in
> > ARCH_DMA_MINALIGN.
> 
> I don't get your conclusion here. Would you mind explaining?

What I wanted to say was that, why ARCH_DMA_MINALIGN should be
different from ARCH_KMALLOC_MINALIGN.

I thought the two were basically same thing. Instead of
decoupling them, I thought just decreasing them in runtime makes more sense.

> > So I'm yet skeptical on decoupling ARCH_DMA/KMALLOC_MINALIGN. Instead
> > of decoupling it, I'm more into dynamically decreasing it.
> 
> The reason for decoupling is mostly that there are some static uses of
> ARCH_KMALLOC_MINALIGN as per point 1 above. The other is the
> __assume_kmalloc_alignment attribute. We shouldn't have such assumed
> alignment larger than what a dynamic kmalloc() would return. To me it
> makes a lot more sense for ARCH_KMALLOC_MINALIGN to be the minimum
> guaranteed in a kernel build but kmalloc() returning a larger alignment
> at run-time than the other way around.

But yeah, considering the problems you mentioned, it seems unavoidable
to decouple them.

Thank you for explanation and I will review slab part soon.

> Thanks.
> 
> -- 
> Catalin

-- 
Thanks,
Hyeonggon