Re: [patch 05/16] mm: Allow special mappings with user access cleared

[Matthew Wilcox]

On Tue, Dec 12, 2017 at 06:32:26PM +0100, Thomas Gleixner wrote:
> From: Peter Zijstra <peterz@xxxxxxxxxxxxx>
> 
> In order to create VMAs that are not accessible to userspace create a new
> VM_NOUSER flag. This can be used in conjunction with
> install_special_mapping() to inject 'kernel' data into the userspace map.

Maybe I misunderstand the intent behind this, but I was recently looking
at something kind of similar.  I was calling it VM_NOTLB and it wouldn't
put TLB entries into the userspace map at all.  The idea was to be able
to use the user address purely as a handle for specific kernel pages,
which were guaranteed to never be mapped into userspace, so we didn't
need to send TLB invalidations when we took those pages away from the user
process again.  But we'd be able to pass the address to read() or write().

So I was going to check the VMA flags in no_page_table() and return the
struct page that was notmapped there.  I didn't get as far as constructing
a prototype yet, and I'm not entirely sure I understand the purpose of
this patch, so perhaps there's no synergy here at all (and perhaps my
idea wouldn't have worked anyway).