Re: [LSF/MM/BPF TOPIC] Page table manipulation primitives

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Feb 06, 2020 at 09:34:10AM -0800, Matthew Wilcox wrote:
> On Thu, Feb 06, 2020 at 06:57:41PM +0200, Mike Rapoport wrote:
> > While updating the architectures to properly use 5-level folded page tables
> > without <asm-generic/?level-fixup.h> and <asm-generic/pgtable-nop4d-hack.h>
> > I wondered if we can do better than explicitly name each and every level of
> > the page table, open-code traversal of all the layers numerous times and
> > have copied do_something_pXd_range().
> > 
> > Then I've come across Kirill's "Proof-of-concept: better(?) page-table
> > manipulation API" [1], but as far as I could see there was no progress
> > since then.
> > 
> > I'd like to resurrect the topic and try to see if we can come up with
> > actually better page table manipulation API.
> > 
> > [1] https://lore.kernel.org/lkml/20180424154355.mfjgkf47kdp2by4e@black.fi.intel.com/

I played a bit more with it after that, but got distracted to other stuff.
I'll see if I'll be able to come up with an update.

> I don't think this approach helps support 64k pages on ARM

Could you specify what such support would require?

> , for example,
> so it doesn't solve enough problems to be worth doing.  I'd favour
> an interface which looked more like this:
> 
> 	vpte_iter iter;
> 	vpte_t vpte;
> 
> 	vpte_iter_for_each(vpte, iter, start, end, flags) {
> 		unsigned char order = vpte_order(&iter);
> 		... do things based on vpte and order ...
> 	}

It looks like just an higher level API that can be provided over my
approach. Maybe it should be the default go-to. But I find it useful to be
able go into low-level details where it is matters.

-- 
 Kirill A. Shutemov