Re: [PATCH 0/7] mm: faster get user pages

[Dave Hansen]

On 09/19/2018 02:02 PM, Keith Busch wrote:
> Pinning user pages out of nvdimm dax memory is significantly slower
> compared to system ram. Analysis points to software overhead incurred
> from a radix tree lookup. This patch series fixes that by removing the
> relatively costly dev_pagemap lookup that was repeated for each page,
> significantly increasing gup time.

Could you also remind us why DAX pages are such special snowflakes and
*require* radix tree lookups in the first place?