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

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Message-ID: <20140519002543.GA3899@node.dhcp.inet.fi>
Date:	Mon, 19 May 2014 03:25:43 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	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?

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.

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

> > +static inline unsigned long vpte_size(vpte_t vptep, enum ptlevel ptlvl)
> > +{
> > +	switch (ptlvl) {
> > +	case PTE:
> > +		return PAGE_SIZE;
> > +#ifdef PMD_SIZE
> > +	case PMD:
> > +		return PMD_SIZE;
> > +#endif
> > +#ifdef PUD_SIZE
> > +	case PUD:
> > +		return PUD_SIZE;
> > +#endif
> > +	default:
> > +		return 0; /* XXX */
> 
> As you say, XXX.  This needs to be an error ... perhaps VM_BUG_ON(1)
> in this case?

Yeah. But include <linux/bug.h> is problematic here...

Anyway the only purpose of the code is to start discussion.

-- 
 Kirill A. Shutemov
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