Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)

[Matthew Wilcox]

On Tue, Nov 14, 2017 at 11:00:20PM -0800, Dave Hansen wrote:
> On 11/14/2017 07:44 PM, Matthew Wilcox wrote:
> > We don't need to kmap in order to access MOVABLE allocations.  kmap is
> > only needed for HIGHMEM allocations.  So there's nothing wrong with ext4
> > or set_bh_page().
> 
> Yeah, it's definitely not _buggy_.
> 
> Although, I do wonder what we should do about these for XPFO.  Should we
> just stick a kmap() in there and comment it?  What we really need is a
> mechanism to say "use this as a kernel page" and "stop using this as a
> kernel page".  kmap() does that... kinda.  It's not a perfect fit, but
> it's pretty close.

It'd be kind of funny if getting XPFO working better means improving
how well Linux runs on 32-bit machines with HIGHMEM.  I think there's
always going to be interest in those -- ARM developed 36 bit physmem
before biting the bullet and going to arm64.  Maybe OpenRISC will do
that next ;-)