Re: [PATCH v2 3/5] mm: Default implementation of arch_wants_pte_order()

[Yu Zhao <yuzhao@google.com>]

On Tue, Jul 4, 2023 at 7:23 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 04/07/2023 13:36, Ryan Roberts wrote:
> > On 04/07/2023 04:59, Yu Zhao wrote:
> >> On Mon, Jul 3, 2023 at 9:02 PM Yu Zhao <yuzhao@google.com> wrote:
> >>>
> >>> On Mon, Jul 3, 2023 at 8:23 PM Yin, Fengwei <fengwei.yin@intel.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 7/3/2023 9:53 PM, Ryan Roberts wrote:
> >>>>> arch_wants_pte_order() can be overridden by the arch to return the
> >>>>> preferred folio order for pte-mapped memory. This is useful as some
> >>>>> architectures (e.g. arm64) can coalesce TLB entries when the physical
> >>>>> memory is suitably contiguous.
> >>>>>
> >>>>> The first user for this hint will be FLEXIBLE_THP, which aims to
> >>>>> allocate large folios for anonymous memory to reduce page faults and
> >>>>> other per-page operation costs.
> >>>>>
> >>>>> Here we add the default implementation of the function, used when the
> >>>>> architecture does not define it, which returns the order corresponding
> >>>>> to 64K.
> >>>>>
> >>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> >>>>> ---
> >>>>>  include/linux/pgtable.h | 13 +++++++++++++
> >>>>>  1 file changed, 13 insertions(+)
> >>>>>
> >>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> >>>>> index a661a17173fa..f7e38598f20b 100644
> >>>>> --- a/include/linux/pgtable.h
> >>>>> +++ b/include/linux/pgtable.h
> >>>>> @@ -13,6 +13,7 @@
> >>>>>  #include <linux/errno.h>
> >>>>>  #include <asm-generic/pgtable_uffd.h>
> >>>>>  #include <linux/page_table_check.h>
> >>>>> +#include <linux/sizes.h>
> >>>>>
> >>>>>  #if 5 - defined(__PAGETABLE_P4D_FOLDED) - defined(__PAGETABLE_PUD_FOLDED) - \
> >>>>>       defined(__PAGETABLE_PMD_FOLDED) != CONFIG_PGTABLE_LEVELS
> >>>>> @@ -336,6 +337,18 @@ static inline bool arch_has_hw_pte_young(void)
> >>>>>  }
> >>>>>  #endif
> >>>>>
> >>>>> +#ifndef arch_wants_pte_order
> >>>>> +/*
> >>>>> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
> >>>>> + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
> >>>>> + * to be at least order-2.
> >>>>> + */
> >>>>> +static inline int arch_wants_pte_order(struct vm_area_struct *vma)
> >>>>> +{
> >>>>> +     return ilog2(SZ_64K >> PAGE_SHIFT);
> >>>> Default value which is not related with any silicon may be: PAGE_ALLOC_COSTLY_ORDER?
> >>>>
> >>>> Also, current pcp list support cache page with order 0...PAGE_ALLOC_COSTLY_ORDER, 9.
> >>>> If the pcp could cover the page, the pressure to zone lock will be reduced by pcp.
> >>>
> >>> The value of PAGE_ALLOC_COSTLY_ORDER is reasonable but again it's a
> >>> s/w policy not a h/w preference. Besides, I don't think we can include
> >>> mmzone.h in pgtable.h.
> >>
> >> I think we can make a compromise:
> >> 1. change the default implementation of arch_has_hw_pte_young() to return 0, and
> >> 2. in memory.c, we can try PAGE_ALLOC_COSTLY_ORDER for archs that
> >> don't override arch_has_hw_pte_young(), or if its return value is too
> >> large to fit.
> >> This should also take care of the regression, right?
> >
> > I think you are suggesting that we use 0 as a sentinel which we then translate
> > to PAGE_ALLOC_COSTLY_ORDER? I already have a max_anon_folio_order() function in
> > memory.c (actually it is currently a macro defined as arch_wants_pte_order()).
> >
> > So it would become (I'll talk about the vma concern separately in the thread
> > where you raised it):
> >
> > static inline int max_anon_folio_order(struct vm_area_struct *vma)
> > {
> >       int order = arch_wants_pte_order(vma);
> >
> >       return order ? order : PAGE_ALLOC_COSTLY_ORDER;
> > }
> >
> > Correct?
>
> Actually, I'm not sure its a good idea to default to a fixed order. If running
> on an arch with big base pages (e.g. powerpc with 64K pages?), that will soon
> add up to a big chunk of memory, which could be wasteful?
>
> PAGE_ALLOC_COSTLY_ORDER = 3 so with 64K base page, that 512K. Is that a concern?
> Wouldn't it be better to define this as an absolute size? Or even the min of
> PAGE_ALLOC_COSTLY_ORDER and an absolute size?

For my POV, not at all. POWER can use smaller page sizes if they
wanted to -- I don't think they do: at least the distros I use on my
POWER9 all have THP=always by default (2MB).