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

[Dan Williams]

On Fri, Dec 14, 2018 at 12:03 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 14, 2018 at 11:53:31AM -0800, Dave Hansen wrote:
> > On 12/14/18 11:48 AM, Matthew Wilcox wrote:
> > > I think we can do better than a proxy object with bit 0 set.  I'd go
> > > for allocating something like this:
> > >
> > > struct dynamic_page {
> > >     struct page;
> > >     unsigned long vaddr;
> > >     unsigned long pfn;
> > >     ...
> > > };
> > >
> > > and use a bit in struct page to indicate that this is a dynamic page.
> >
> > That might be fun.  We'd just need a fast/static and slow/dynamic path
> > in page_to_pfn()/pfn_to_page().  We'd also need some kind of auxiliary
> > pfn-to-page structure since we could not fit that^ structure in vmemmap[].
>
> 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:

    df06b37ffe5a mm/gup: cache dev_pagemap while pinning pages