Re: [PATCHv3 10/11] arm64: Add 16K page size support

[Suzuki K. Poulose]

On 15/10/15 15:06, Mark Rutland wrote:
> Hi,

I have fixed all the nits locally. Thanks for pointing them out.

> >   config FORCE_MAX_ZONEORDER
> >   	int
> >   	default "14" if (ARM64_64K_PAGES && TRANSPARENT_HUGEPAGE)
> > +	default "12" if (ARM64_16K_PAGES && TRANSPARENT_HUGEPAGE)
> >   	default "11"
>
> I'm a little lost here. How are these numbers derived?

I struggled to find the right value for 16K. Thanks to Steve Capper
for the following explanation. I will add it as a comment.

All allocations from the buddy allocator have to have compound order
strictly less than MAX_ORDER. i.e, the maximum allocation size is
(MAX_ORDER - 1) PAGES. To align with the transparent huge page size,
we get :

 (MAX_ORDER - 1) + PAGE_SHIFT = PMD_SHIFT

Which gives us:

MAX_ORDER = PAGE_SHIFT - 3 + PAGE_SHIFT - PAGE_SHIFT + 1
          = PAGE_SHIFT - 2

That raises an interesting question about the selection of the value
for 4K. Shouldn't that be 10 instead of 11 ?

Steve ?

> > -#ifdef CONFIG_ARM64_64K_PAGES
> > +#if	defined(CONFIG_ARM64_64K_PAGES)
> >   #define NR_FIX_BTMAPS		4
> > +#elif	defined (CONFIG_ARM64_16K_PAGES)
> > +#define NR_FIX_BTMAPS		16
> >   #else
> >   #define NR_FIX_BTMAPS		64
> >   #endif
>
> We could include <linux/sizes.h> and simplify this to:
>
> #define NR_FIX_BTMAPS (SZ_256K / PAGE_SIZE)
>
> Which works for me locally.

Nice cleanup. I will pick that as a separate patch in the series.

> > diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> > index 5eac6a2..90c7ff2 100644
> > --- a/arch/arm64/include/asm/thread_info.h
> > +++ b/arch/arm64/include/asm/thread_info.h
> > @@ -25,6 +25,8 @@
> >
> >   #ifdef CONFIG_ARM64_4K_PAGES
> >   #define THREAD_SIZE_ORDER	2
> > +#elif defined(CONFIG_ARM64_16K_PAGES)
> > +#define THREAD_SIZE_ORDER	0
> >   #endif
> >   #define THREAD_SIZE		16384
>
> The above looks correct.
>
> As an open/general question, why do both THREAD_SIZE_ORDER and
> THREAD_SIZE exist? One really should be defined in terms of the other.

I think its mainly for choosing the mechanism for stack allocation. If it
is a multiple of a page, you allocate a page. If not, uses a kmem_cache.


> >   #define id_aa64mmfr0_tgran_shift	ID_AA64MMFR0_TGRAN4_SHIFT
> >   #define id_aa64mmfr0_tgran_on		ID_AA64MMFR0_TGRAN4_ON
>
> I assume you'll s/ON/SUPPORTED/ per comments in another thread.

Yes

Thanks
Suzuki

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