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

[Keith Busch]

On Wed, Sep 19, 2018 at 02:15:28PM -0700, Dave Hansen wrote:
> 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?

Yeah, ZONE_DEVICE memory is special. It has struct page mappings, but
not for online general use. The dev_pagemap is the metadata to the zone
device memory, and that metadata is stashed in a radix tree.

We're looking up the dev_pagemap in this path to take a reference so
the zone device can't be unmapped.