Re: [PATCH v1 08/10] mm: Kconfig hooks to determine max anon folio allocation order

Ryan Roberts
2023-06-27T09:54:08Z

On 27/06/2023 03:47, Yu Zhao wrote:
> On Mon, Jun 26, 2023 at 11:15 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> For variable-order anonymous folios, we need to determine the order that
>> we will allocate. From a SW perspective, the higher the order we
>> allocate, the less overhead we will have; fewer faults, fewer folios in
>> lists, etc. But of course there will also be more memory wastage as the
>> order increases.
>>
>> From a HW perspective, there are memory block sizes that can be
>> beneficial to reducing TLB pressure. arm64, for example, has the ability
>> to map "contpte" sized chunks (64K for a 4K base page, 2M for 16K and
>> 64K base pages) such that one of these chunks only uses a single TLB
>> entry.
>>
>> So we let the architecture specify the order of the maximally beneficial
>> mapping unit when PTE-mapped. Furthermore, because in some cases, this
>> order may be quite big (and therefore potentially wasteful of memory),
>> allow the arch to specify 2 values; One is the max order for a mapping
>> that _would not_ use THP if all size and alignment constraints were met,
>> and the other is the max order for a mapping that _would_ use THP if all
>> those constraints were met.
>>
>> Implement this with Kconfig by introducing some new options to allow the
>> architecture to declare that it supports large anonymous folios along
>> with these 2 preferred max order values. Then introduce a user-facing
>> option, LARGE_ANON_FOLIO, which defaults to disabled and can only be
>> enabled if the architecture has declared its support. When disabled, it
>> forces the max order values, LARGE_ANON_FOLIO_NOTHP_ORDER_MAX and
>> LARGE_ANON_FOLIO_THP_ORDER_MAX to 0, meaning only a single page is ever
>> allocated.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>> mm/Kconfig | 39 +++++++++++++++++++++++++++++++++++++++
>> mm/memory.c | 8 ++++++++
>> 2 files changed, 47 insertions(+)
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index 7672a22647b4..f4ba48c37b75 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -1208,4 +1208,43 @@ config PER_VMA_LOCK
>>
>> source "mm/damon/Kconfig"
>>
>> +config ARCH_SUPPORTS_LARGE_ANON_FOLIO
>> + def_bool n
>> + help
>> + An arch should select this symbol if wants to allow LARGE_ANON_FOLIO
>> + to be enabled. It must also set the following integer values:
>> + - ARCH_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX
>> + - ARCH_LARGE_ANON_FOLIO_THP_ORDER_MAX
>> +
>> +config ARCH_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX
>> + int
>> + help
>> + The maximum size of folio to allocate for an anonymous VMA PTE-mapping
>> + that does not have the MADV_HUGEPAGE hint set.
>> +
>> +config ARCH_LARGE_ANON_FOLIO_THP_ORDER_MAX
>> + int
>> + help
>> + The maximum size of folio to allocate for an anonymous VMA PTE-mapping
>> + that has the MADV_HUGEPAGE hint set.
>> +
>> +config LARGE_ANON_FOLIO
>> + bool "Allocate large folios for anonymous memory"
>> + depends on ARCH_SUPPORTS_LARGE_ANON_FOLIO
>> + default n
>> + help
>> + Use large (bigger than order-0) folios to back anonymous memory where
>> + possible. This reduces the number of page faults, as well as other
>> + per-page overheads to improve performance for many workloads.
>> +
>> +config LARGE_ANON_FOLIO_NOTHP_ORDER_MAX
>> + int
>> + default 0 if !LARGE_ANON_FOLIO
>> + default ARCH_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX
>> +
>> +config LARGE_ANON_FOLIO_THP_ORDER_MAX
>> + int
>> + default 0 if !LARGE_ANON_FOLIO
>> + default ARCH_LARGE_ANON_FOLIO_THP_ORDER_MAX
>> +
>> endmenu
>
> I don't think an MVP should add this many Kconfigs. One Kconfig sounds
> reasonable to me for now.

If we move to arch_wants_pte_order() as you suggested (in your response to patch
3) then I agree we can remove most of these. I still think we might want 2
though. For an arch that does not implement arch_wants_pte_order() we wouldn't
want LARGE_ANON_FOLIO to show up in menuconfig so we would still need
ARCH_SUPPORTS_LARGE_ANON_FOLIO:

config ARCH_SUPPORTS_LARGE_ANON_FOLIO
def_bool n
help
An arch should select this symbol if wants to allow LARGE_ANON_FOLIO
to be enabled. In this case, It must also define arch_wants_pte_order()

config LARGE_ANON_FOLIO
bool "Allocate large folios for anonymous memory"
depends on ARCH_SUPPORTS_LARGE_ANON_FOLIO
default n
help
Use large (bigger than order-0) folios to back anonymous memory where
possible. This reduces the number of page faults, as well as other
per-page overheads to improve performance for many workloads.

What do you think?