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Date:   Fri, 4 May 2018 14:12:44 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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 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.  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.

> +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.

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

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