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

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Message-ID: <1400171258.21547.136.camel@deneb.redhat.com>
Date:	Thu, 15 May 2014 12:27:38 -0400
From:	Mark Salter <msalter@...hat.com>
To:	Steve Capper <steve.capper@...aro.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: fix pud_huge() for 2-level pagetables

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:
> > The following happens when trying to run a kvm guest on a kernel
> > configured for 64k pages. This doesn't happen with 4k pages:
> > 
> >   BUG: failure at include/linux/mm.h:297/put_page_testzero()!
> >   Kernel panic - not syncing: BUG!
> >   CPU: 2 PID: 4228 Comm: qemu-system-aar Tainted: GF            3.13.0-0.rc7.31.sa2.k32v1.aarch64.debug #1
> >   Call trace:
> >   [<fffffe0000096034>] dump_backtrace+0x0/0x16c
> >   [<fffffe00000961b4>] show_stack+0x14/0x1c
> >   [<fffffe000066e648>] dump_stack+0x84/0xb0
> >   [<fffffe0000668678>] panic+0xf4/0x220
> >   [<fffffe000018ec78>] free_reserved_area+0x0/0x110
> >   [<fffffe000018edd8>] free_pages+0x50/0x88
> >   [<fffffe00000a759c>] kvm_free_stage2_pgd+0x30/0x40
> >   [<fffffe00000a5354>] kvm_arch_destroy_vm+0x18/0x44
> >   [<fffffe00000a1854>] kvm_put_kvm+0xf0/0x184
> >   [<fffffe00000a1938>] kvm_vm_release+0x10/0x1c
> >   [<fffffe00001edc1c>] __fput+0xb0/0x288
> >   [<fffffe00001ede4c>] ____fput+0xc/0x14
> >   [<fffffe00000d5a2c>] task_work_run+0xa8/0x11c
> >   [<fffffe0000095c14>] do_notify_resume+0x54/0x58
> > 
> > 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.
> 
> Hi Mark,
> I'm still catching up with myself, sorry  (was off sick for a couple
> of days)...
> 
> I thought unmap_range was going to be changed?
> Does the following help things?
> https://lists.cs.columbia.edu/pipermail/kvmarm/2014-May/009388.html

No, I get the same BUG. Regardless, pud_huge() should always return
false for 2-level pagetables, right?


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