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

[Joao Martins <joao.m.martins@oracle.com>]

On 5/6/21 9:05 AM, Aneesh Kumar K.V wrote:
> 
> 
> IIUC this series is about devdax namespace with aligh of 1G or 2M where we can
> save the vmmemap space by not allocating memory for tail struct pages? 
> 
Right.

It reuses base pages across the vmemmap, but for the base pages containing
only the tail struct pages.

> Dan Williams <dan.j.williams@intel.com> writes:
> 
>>> enum:
>>>>
>>>> enum devmap_geometry {
>>>>     DEVMAP_PTE,
>>>>     DEVMAP_PMD,
>>>>     DEVMAP_PUD,
>>>> }
>>>>
>>> I suppose a converter between devmap_geometry and page_size would be needed too? And maybe
>>> the whole dax/nvdimm align values change meanwhile (as a followup improvement)?
>>
>> 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.
* namespaces with align of 1G would use *compound struct pages of order-21* (DEVMAP_PMD)
backed with PTE entries in the direct map.