linux-kernel - Re: [RFC, PATCH] mm: unified interface to handle page table entries on different levels?

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Message-ID: <87fvk5k51b.fsf@linux.vnet.ibm.com>
Date:	Mon, 19 May 2014 23:46:32 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Matthew Wilcox <willy@...ux.intel.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, dave@...1.net,
	riel@...hat.com, mgorman@...e.de, aarcange@...hat.com
Subject: Re: [RFC, PATCH] mm: unified interface to handle page table entries on different levels?

"Kirill A. Shutemov" <kirill@...temov.name> writes:

> On Sun, May 18, 2014 at 07:45:59PM -0400, Matthew Wilcox wrote:
>> On Sat, May 17, 2014 at 03:33:05AM +0300, Kirill A. Shutemov wrote:
>> > Below is my attempt to play with the problem. I've took one function --
>> > page_referenced_one() -- which looks ugly because of different APIs for
>> > PTE/PMD and convert it to use vpte_t. vpte_t is union for pte_t, pmd_t
>> > and pud_t.
>> > 
>> > Basically, the idea is instead of having different helpers to handle
>> > PTE/PMD/PUD, we have one, which take pair of vpte_t + pglevel.
>> 
>> I can't find my original attempt at this now (I am lost in a maze of
>> twisted git trees, all subtly different), but I called it a vpe (Virtual
>> Page Entry).
>> 
>> Rather than using a pair of vpte_t and pglevel, the vpe_t contained
>> enough information to discern what level it was; that's only two bits
>> and I think all the architectures have enough space to squeeze in two
>> more bits to the PTE (the PMD and PUD obviously have plenty of space).
>
> I'm not sure if it's possible to find a single free bit on all
> architectures. Two is near impossible.

On ppc64 we don't have any free bits.

>
> And what about 5-level page tables in future? Will we need 3 bits there?
> No way.

-aneesh

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