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SubjectRe: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files
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On 12/12/19 8:07 PM, Liran Alon wrote:
>> I was a little hesitant to change the this to handle 1 GB pages with this patchset at first. I didn't want to break the non-DAX case stuff by doing so.
>
> Why would it affect non-DAX case?
> Your patch should just make hugepage_adjust() to parse page-tables only in case is_zone_device_page(). Otherwise, page tables shouldn’t be parsed.
> i.e. THP merged pages should still be detected by PageTransCompoundMap().

That's what I already do. But if I wanted to make the hugepage_adjust()
function also handle the change to 1 GB, then that code would apply to
THP too. I didn't want to do that without knowing the implications for THP.

>> Specifically, can a THP page be 1 GB, and if so, how can you tell? If you can't tell easily, I could walk the page table for all cases, instead of just zone_device().
>
> I prefer to walk page-tables only for is_zone_device_page().

Is there another way to tell if a THP page is 1 GB? Anyway, this is the
sort of stuff I didn't want to mess around with.

hugepage_adjust() seemed like a reasonable place to get a huge (2MB)
page table entry out of a DAX mapping. I didn't want to proliferate
another special case for upgrading to a larger PTE size (i.e. how
hugetlbfs and THP have separate mechanisms), so I hopped on to the "can
we do a 2MB mapping even though host_mapping_level() didn't say so" case
- which is my interpretation of what huge_adjust() is for.

Barret


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