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

[Suzuki K. Poulose]

On 15/10/15 16:36, Steve Capper wrote:
> On 15 October 2015 at 15:48, Suzuki K. Poulose <Suzuki.Poulose@xxxxxxx> wrote:
> > 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 ?
>
> Hi,
> My understanding is that 11 is a "good minimum" value for the page
> allocator with 4KB pages.
> (There are references to it being 10 in 2.4 kernels but raised to 11
> on 2.6 kernels?)
>
> We need to raise the minimum when we have a 16KB or 64KB PAGE_SIZE to
> be able allocate a 32MB or 512MB Transparent HugePages.

Thanks Steve, for the clarification. I will add the following comment
to the Kconfig

#
# 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
#
# However for 4K, we choose a higher default value 11 as opposed to 10, (giving us size 4M)
# matching the default value used by the generic code.
#


Thanks
Suzuki

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