Re: [RFC v2 PATCH 16/17] mm: mmap: Align unhinted maps to highest anon folio order

Ryan Roberts
2023-04-17T10:13:16Z

On 17/04/2023 09:25, Yin, Fengwei wrote:
>
>
> On 4/14/2023 9:03 PM, Ryan Roberts wrote:
>> When allocating large anonymous folios, we want to maximize our chances
>> of being able to use the highest order we support. Since one of the
>> constraints is that a folio has to be mapped naturally aligned, let's
>> have mmap default to that alignment when user space does not provide a
>> hint.
>>
>> With this in place, an extra 2% of all allocated anonymous memory
>> belongs to a folio of the highest order, when compiling the kernel.
> This change has side effect: reduce the chance of VMA merging.
> So benefit to per-VMA lock also. But find VMA need searching more VMAs.

Good point. This change brings only a very marginal benefit anyway, so I think I
might just drop the it from the series to avoid any unexpected issues.

>
>
> Regards
> Yin, Fengwei
>
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>> mm/mmap.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index ff68a67a2a7c..e7652001a32e 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -1627,7 +1627,7 @@ generic_get_unmapped_area(struct file *filp, unsigned long addr,
>> info.length = len;
>> info.low_limit = mm->mmap_base;
>> info.high_limit = mmap_end;
>> - info.align_mask = 0;
>> + info.align_mask = BIT(PAGE_SHIFT + ANON_FOLIO_ORDER_MAX) - 1;
>> info.align_offset = 0;
>> return vm_unmapped_area(&info);
>> }
>> @@ -1677,7 +1677,7 @@ generic_get_unmapped_area_topdown(struct file *filp, unsigned long addr,
>> info.length = len;
>> info.low_limit = max(PAGE_SIZE, mmap_min_addr);
>> info.high_limit = arch_get_mmap_base(addr, mm->mmap_base);
>> - info.align_mask = 0;
>> + info.align_mask = BIT(PAGE_SHIFT + ANON_FOLIO_ORDER_MAX) - 1;
>> info.align_offset = 0;
>> addr = vm_unmapped_area(&info);
>>
>> --
>> 2.25.1
>>