Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Matthew Wilcox]

On Fri, Dec 14, 2018 at 12:17:08PM -0800, Dan Williams wrote:
> On Fri, Dec 14, 2018 at 12:03 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > Yes; working on the pfn-to-page structure right now as it happens ...
> > in the meantime, an XArray for it probably wouldn't be _too_ bad.
> 
> It might... see the recent patch from Ketih responding to complaints
> about get_dev_pagemap() lookup overhead:

Yeah, I saw.  I called xa_dump() on the pgmap_array() running under
QEmu and it's truly awful because the NVDIMMs presented by QEmu are
very misaligned.  If we can make the NVDIMMs better aligned, we won't
hit such a bad case in the XArray data structure.