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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 13, 2017 at 11:12:33PM +0100, Peter Zijlstra wrote:
> On Wed, Dec 13, 2017 at 01:50:22PM -0800, Matthew Wilcox wrote:
> > On Tue, Dec 12, 2017 at 06:32:26PM +0100, Thomas Gleixner wrote:
> > > From: Peter Zijstra <peterz@infradead.org>
> > > 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().
> 
> Since the LDT is strictly per process, the idea was to actually inject
> it into the userspace map. Except of course, userspace must not actually
> be able to access it. So by mapping it !_PAGE_USER its 'invisible'.
> 
> But the CPU very much needs the mapping, it will load the LDT entries
> through them.

So can I use your VM_NOUSER VMAs for my purpose?  That is, can I change
the page table without flushing the TLB?  The only access to these PTEs
will be through the kernel mapping, so I don't see why I'd need to.