Re: [PATCH v2 1/2] x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems

From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
	X86 ML <x86@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Neil Berrington <neil.berrington@datacore.com>,
	LKML <linux-kernel@vger.kernel.org>,
	stable <stable@vger.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v2 1/2] x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems
Date: Fri, 26 Jan 2018 10:30:07 +0100	[thread overview]
Message-ID: <20180126093006.n55q4dkkrexymsji@gmail.com> (raw)
In-Reply-To: <CALCETrVwJCL=QTRT70b8u3p8xOXUiC7_Mkz45Bi3M9-vYgXWtg@mail.gmail.com>

* Andy Lutomirski <luto@kernel.org> wrote:

> What I'd really like to see is an entirely different API.  Maybe:
> 
> typedef struct {
>   opaque, but probably includes:
>   int depth;  /* 0 is root */
>   void *table;
> } ptbl_ptr;
> 
> ptbl_ptr root_table = mm_root_ptbl(mm);
> 
> set_ptbl_entry(root_table, pa, prot);
> 
> /* walk tables */
> ptbl_ptr pt = ...;
> ptentry_ptr entry;
> while (ptbl_has_children(pt)) {
>   pt = pt_next(pt, addr);
> }
> entry = pt_entry_at(pt, addr);
> /* do something with entry */
> 
> etc.
> 
> Now someone can add a sixth level without changing every code path in
> the kernel that touches page tables.

Iteration based page table lookups would be neat.

A sixth level is unavoidable on x86-64 I think - we'll get there in a decade or 
so? The sixth level will also use up the last ~8 bits of virtual memory available 
on 64-bit.

Thanks,

	Ingo