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 <[email protected]> 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.
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