linux-kernel - Re: [PATCH] arm64: fix pud_huge() for 2-level pagetables

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Date:	Fri, 16 May 2014 10:51:25 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	"msalter@...hat.com" <msalter@...hat.com>
Cc:	Steve Capper <steve.capper@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: fix pud_huge() for 2-level pagetables

On Thu, May 15, 2014 at 07:39:17PM +0100, Mark Salter wrote:
> On Thu, 2014-05-15 at 18:55 +0100, Steve Capper wrote:
> > On 15 May 2014 17:27, Mark Salter <msalter@...hat.com> wrote:
> > > On Thu, 2014-05-15 at 15:44 +0100, Steve Capper wrote:
> > >> On Thu, May 15, 2014 at 10:19:22AM -0400, Mark Salter wrote:
> > >> > In arch/arm/kvm/mmu.c:unmap_range(), we end up doing an extra put_page()
> > >> > on the stage2 pgd which leads to the BUG in put_page_testzero(). This
> > >> > happens because a pud_huge() test in unmap_range() returns true when it
> > >> > should always be false with 2-level pages tables used by 64k pages.
> > >> > This patch removes support for huge puds if 2-level pagetables are
> > >> > being used.
[...]
> > Yeah I agree for 64K granule it doesn't make sense to have a huge_pud.
> > The patch looks sound now, but checking for a folded pmd may run into
> > problems if/when we get to 3-levels and 64K pages in future.
> > 
> > Perhaps checking for PAGE_SHIFT==12 (or something similar) would be a
> > bit more robust?
> 
> I don't think testing based on granule size is generally correct either.
> Maybe support for 3-level page tables with 64k granule gets added as an
> option. That would break the pagesize based test. With a folded pmd, we
> know there is no pud, so pud_huge() should always be false.

I agree, pud_huge() should be false in the same way we define
pud_present() to be 1 when __PGTABLE_PMD_FOLDED. The *_huge() macros
aren't covered by the generic headers unfortunately (some clean-up would
be useful at some point but for now this patch is fine).

-- 
Catalin
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