linux-kernel - Re: [PATCH 2/2] KVM: arm64: Only reschedule if MMU_NOTIFIER_RANGE

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Date:   Tue, 11 Aug 2020 11:52:41 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     will@...nel.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     maz@...nel.org, james.morse@....com, tsbogend@...ha.franken.de,
        paulus@...abs.org, pbonzini@...hat.com,
        sean.j.christopherson@...el.com, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: arm64: Only reschedule if
 MMU_NOTIFIER_RANGE_BLOCKABLE is not set

On 08/11/2020 11:27 AM, Will Deacon wrote:
> When an MMU notifier call results in unmapping a range that spans multiple
> PGDs, we end up calling into cond_resched_lock() when crossing a PGD boundary,
> since this avoids running into RCU stalls during VM teardown. Unfortunately,
> if the VM is destroyed as a result of OOM, then blocking is not permitted
> and the call to the scheduler triggers the following BUG():
> 
>   | BUG: sleeping function called from invalid context at arch/arm64/kvm/mmu.c:394
>   | in_atomic(): 1, irqs_disabled(): 0, non_block: 1, pid: 36, name: oom_reaper
>   | INFO: lockdep is turned off.
>   | CPU: 3 PID: 36 Comm: oom_reaper Not tainted 5.8.0 #1
>   | Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
>   | Call trace:
>   |  dump_backtrace+0x0/0x284
>   |  show_stack+0x1c/0x28
>   |  dump_stack+0xf0/0x1a4
>   |  ___might_sleep+0x2bc/0x2cc
>   |  unmap_stage2_range+0x160/0x1ac
>   |  kvm_unmap_hva_range+0x1a0/0x1c8
>   |  kvm_mmu_notifier_invalidate_range_start+0x8c/0xf8
>   |  __mmu_notifier_invalidate_range_start+0x218/0x31c
>   |  mmu_notifier_invalidate_range_start_nonblock+0x78/0xb0
>   |  __oom_reap_task_mm+0x128/0x268
>   |  oom_reap_task+0xac/0x298
>   |  oom_reaper+0x178/0x17c
>   |  kthread+0x1e4/0x1fc
>   |  ret_from_fork+0x10/0x30
> 
> Use the new 'flags' argument to kvm_unmap_hva_range() to ensure that we
> only reschedule if MMU_NOTIFIER_RANGE_BLOCKABLE is set in the notifier
> flags.
> 
> Cc: <stable@...r.kernel.org>
> Fixes: 8b3405e345b5 ("kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd")
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Cc: James Morse <james.morse@....com>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>