Re: [RFC v2 PATCH 05/17] mm: Routines to determine max anon folio allocation order

[Ryan Roberts <ryan.roberts@arm.com>]

On 14/04/2023 15:09, Kirill A. Shutemov wrote:
> On Fri, Apr 14, 2023 at 02:02:51PM +0100, Ryan Roberts wrote:
>> For variable-order anonymous folios, we want to tune the order that we
>> prefer to allocate based on the vma. Add the routines to manage that
>> heuristic.
>>
>> TODO: Currently we always use the global maximum. Add per-vma logic!
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>  include/linux/mm.h | 5 +++++
>>  mm/memory.c        | 8 ++++++++
>>  2 files changed, 13 insertions(+)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index cdb8c6031d0f..cc8d0b239116 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3674,4 +3674,9 @@ madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
>>  }
>>  #endif
>>
>> +/*
>> + * TODO: Should this be set per-architecture?
>> + */
>> +#define ANON_FOLIO_ORDER_MAX	4
>> +
> 
> I think it has to be derived from size in bytes, not directly specifies
> page order. For 4K pages, order 4 is 64k and for 64k pages it is 1M.
> 

Yes I see where you are coming from. What's your feel for what a sensible upper
bound in bytes is?

My difficulty is that I would like to be able to use this allocation mechanism
to enable using the "contiguous bit" on arm64; that's a set of contiguous PTEs
that are mapped to physically contiguous memory, and the HW can use that hint to
coalesce the TLB entries.

For 4KB pages, the contig size is 64KB (order-4), so that works nicely. But for
16KB and 64KB pages, its 2MB (order-7 and order-5 respectively). Do you think
allocating 2MB pages here is going to lead to too much memory wastage?