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Date:   Sat, 5 May 2018 00:49:36 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>, x86@kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: Proof-of-concept: better(?) page-table manipulation API
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On Fri, May 04, 2018 at 09:12:44PM +0000, Matthew Wilcox wrote:
> On Tue, Apr 24, 2018 at 06:43:56PM +0300, Kirill A. Shutemov wrote:
> > +struct pt_ptr {
> > +	unsigned long *ptr;
> > +	int lvl;
> > +};
> 
> On x86, you've got three kinds of paging scheme, referred to in the manual
> as 32-bit, PAE and 4-level.

You forgot 5-level :)

(although it's not in the manual yet, so fair enough)

> On 32-bit, you've got 3 levels (Directory, Table and Entry), and you can
> encode those three levels in the bottom two bits of the pointer.  With
> PAE and 4L, pointers are 64-bit aligned, so you can encode up to eight
> levels in the bottom three bits of the pointer.

I didn't thought about this. Thank you.

> > +struct pt_val {
> > +	unsigned long val;
> > +	int lvl;
> > +};
> 
> I don't think it's possible to shrink this down to a single ulong.
> _Maybe_ it is if you can squirm a single bit free from the !pte_present
> case.

I don't think it worth it. It gets tricky quickly.

> ... this is only for x86 4L and maybe 32 paging, right?  It'd need to
> use unsigned long val[2] for PAE.

I didn't look at 32-bit at all. But 4L and 5L [kinda] work.

> I'm going to think about this some more.  There's a lot of potential here.

Thanks for the input.

-- 
 Kirill A. Shutemov