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

[Mark Salter]

On Thu, 2014-05-15 at 18:55 +0100, Steve Capper wrote:
> On 15 May 2014 17:27, Mark Salter <msalter@xxxxxxxxxx> 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:
> >> > 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?
> 
> Okay, thanks for giving that a go.
> 
> 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.



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