linux-kernel - Re: [PATCH stable-4.14.y backport] KVM: arm/arm64: Don't reschedule in unmap_stage2

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Message-ID: <2d18375b13eeae3ae12dda5393154857@kernel.org>
Date:   Mon, 24 Aug 2020 13:56:18 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        suzuki.poulose@....com, james.morse@....com, pbonzini@...hat.com,
        kernel-team@...roid.com, stable@...r.kernel.org
Subject: Re: [PATCH stable-4.14.y backport] KVM: arm/arm64: Don't reschedule
 in unmap_stage2_range()

On 2020-08-24 12:29, Will Deacon wrote:
> Upstream commits fdfe7cbd5880 ("KVM: Pass MMU notifier range flags to
> kvm_unmap_hva_range()") and b5331379bc62 ("KVM: arm64: Only reschedule
> if MMU_NOTIFIER_RANGE_BLOCKABLE is not set") fix a "sleeping from 
> invalid
> context" BUG caused by unmap_stage2_range() attempting to reschedule 
> when
> called on the OOM path.
> 
> Unfortunately, these patches rely on the MMU notifier callback being
> passed knowledge about whether or not blocking is permitted, which was
> introduced in 4.19. Rather than backport this considerable amount of
> infrastructure just for KVM on arm, instead just remove the conditional
> reschedule.
> 
> Cc: <stable@...r.kernel.org> # v4.14 only
> 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>
> ---
>  virt/kvm/arm/mmu.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 3814cdad643a..7fe673248e98 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -307,12 +307,6 @@ static void unmap_stage2_range(struct kvm *kvm,
> phys_addr_t start, u64 size)
>  		next = stage2_pgd_addr_end(addr, end);
>  		if (!stage2_pgd_none(*pgd))
>  			unmap_stage2_puds(kvm, pgd, addr, next);
> -		/*
> -		 * If the range is too large, release the kvm->mmu_lock
> -		 * to prevent starvation and lockup detector warnings.
> -		 */
> -		if (next != end)
> -			cond_resched_lock(&kvm->mmu_lock);
>  	} while (pgd++, addr = next, addr != end);
>  }

Acked-by: Marc Zyngier <maz@...nel.org>

         M.
-- 
Jazz is not dead. It just smells funny...