Re: [PATCH v1 04/11] mm/memremap: add ZONE_DEVICE support for compound pages

Joao Martins
2021-05-06T12:16:03Z

On 5/6/21 12:43 PM, Matthew Wilcox wrote:
> On Thu, May 06, 2021 at 11:23:25AM +0100, Joao Martins wrote:
>>>> I think it is ok for dax/nvdimm to continue to maintain their align
>>>> value because it should be ok to have 4MB align if the device really
>>>> wanted. However, when it goes to map that alignment with
>>>> memremap_pages() it can pick a mode. For example, it's already the
>>>> case that dax->align == 1GB is mapped with DEVMAP_PTE today, so
>>>> they're already separate concepts that can stay separate.
>>>
>>> devdax namespace with align of 1G implies we expect to map them with 1G
>>> pte entries? I didn't follow when you say we map them today with
>>> DEVMAP_PTE entries.
>>
>> This sort of confusion is largelly why Dan is suggesting a @geometry for naming rather
>> than @align (which traditionally refers to page tables entry sizes in pagemap-related stuff).
>>
>> DEVMAP_{PTE,PMD,PUD} refers to the representation of metadata in base pages (DEVMAP_PTE),
>> compound pages of PMD order (DEVMAP_PMD) or compound pages of PUD order (DEVMAP_PUD).
>>
>> So, today:
>>
>> * namespaces with align of 1G would use *struct pages of order-0* (DEVMAP_PTE) backed with
>> PMD entries in the direct map.
>> * namespaces with align of 2M would use *struct pages of order-0* (DEVMAP_PTE) backed with
>> PMD entries in the direct map.
>>
>> After this series:
>>
>> * namespaces with align of 1G would use *compound struct pages of order-30* (DEVMAP_PUD)
>> backed with PMD entries in the direct map.
>
> order-18
>
Right, thanks for the correction.

Forgot to subtract PAGE_SIZE order (12).

>> * namespaces with align of 1G would use *compound struct pages of order-21* (DEVMAP_PMD)
>> backed with PTE entries in the direct map.
>
> i think you mean "align of 2M", and order-9.
>
True.

> (note that these numbers are/can be different for powerpc since it
> supports different TLB page sizes. please don't get locked into x86
> sizes, and don't ignore the benefits *to software* of managing memory
> in sizes other than just those supported by the hardware).
>
I am not ignoring that either that I think.

The page size looking values above is also what the consumer of this (device-dax) allows
you to use as @align, hence why you see DEVMAP_PTE, PMD and PUD as the sizes.