On Thu, 2014-05-15 at 18:55 +0100, Steve Capper wrote: > On 15 May 2014 17:27, Mark Salter <msal...@redhat.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: > >> > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
