When evincting pages from the host, we may end up in a situation where a single page is getting evicted on multiple CPUs at the same time. Because we serialize the evictions by holding pgd_lock, we make sure that all CPUs but the first one will see the page as already gone. If that page was the last in a section, the section is removed as well. And all other CPUs will hit the BUG_ON condition, like this: [ 312.189234] ------------[ cut here ]------------ [ 312.203056] kernel BUG at arch/arm/kvm/mmu.c:382! [ 312.217134] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP THUMB2 [ 312.235376] Modules linked in: [ 312.244515] CPU: 0 Not tainted (3.6.0-rc3+ #40) [ 312.259118] PC is at stage2_clear_pte+0x128/0x134 [ 312.273193] LR is at kvm_unmap_hva+0x97/0xa0 [ 312.285967] pc : [<c001e10c>] lr : [<c001ee0f>] psr: 60000133 [ 312.285967] sp : caa25998 ip : df97a028 fp : 00800000 [ 312.320355] r10: 873b5b5f r9 : c8654000 r8 : 01c55000 [ 312.335990] r7 : 00000000 r6 : df249c00 r5 : c688fb80 r4 : df249ccc [ 312.355532] r3 : 00000000 r2 : 2e001000 r1 : 00000000 r0 : 00000000 [ 312.375076] Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA Thumb Segment user [ 312.396962] Control: 70c5387d Table: 8a9bbb00 DAC: fffffffd [ 312.414161] Process hackbench (pid: 7207, stack limit = 0xcaa242f8) It is actually safe to remove the BUG_ONs and just return from the function earlier. Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx> --- arch/arm/kvm/mmu.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c index e76c6a0..3610472 100644 --- a/arch/arm/kvm/mmu.c +++ b/arch/arm/kvm/mmu.c @@ -376,10 +376,12 @@ static void stage2_clear_pte(struct kvm *kvm, phys_addr_t addr) pgd = kvm->arch.pgd + pgd_index(addr); pud = pud_offset(pgd, addr); - BUG_ON(pud_none(*pud)); + if (pud_none(*pud)) + return; /* Probably evicted already by another CPU */ pmd = pmd_offset(pud, addr); - BUG_ON(pmd_none(*pmd)); + if (pmd_none(*pmd)) + return; /* Probably evicted already by another CPU */ pte = pte_offset_kernel(pmd, addr); set_pte_ext(pte, __pte(0), 0); -- 1.7.12 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/cucslists/listinfo/kvmarm
