linux-kernel - Re: [PATCH] KVM: arm/arm64: add WARN_ON if size is not PAGE_SIZE aligned in unmap_stage2

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Date:   Thu, 17 May 2018 09:17:44 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Jia He <hejianet@...il.com>,
        Christoffer Dall <christoffer.dall@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Minchan Kim <minchan@...nel.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jia.he@...-semitech.com
Subject: Re: [PATCH] KVM: arm/arm64: add WARN_ON if size is not PAGE_SIZE
 aligned in unmap_stage2_range


Hi Jia,

On 17/05/18 07:11, Jia He wrote:
> I ever met a panic under memory pressure tests(start 20 guests and run
> memhog in the host).

Please avoid using "I" in the commit description and preferably stick to
an objective description.

> 
> The root cause might be what I fixed at [1]. But from arm kvm points of
> view, it would be better we caught the exception earlier and clearer.
> 
> If the size is not PAGE_SIZE aligned, unmap_stage2_range might unmap the
> wrong(more or less) page range. Hence it caused the "BUG: Bad page
> state"

I don't see why we should ever panic with a "positive" size value. Anyways,
the unmap requests must be in units of pages. So this check might be useful.


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

> 
> [1] https://lkml.org/lkml/2018/5/3/1042
> 
> Signed-off-by: jia.he@...-semitech.com
> ---
>   virt/kvm/arm/mmu.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 7f6a944..8dac311 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -297,6 +297,8 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
>   	phys_addr_t next;
>   
>   	assert_spin_locked(&kvm->mmu_lock);
> +	WARN_ON(size & ~PAGE_MASK);
> +
>   	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
>   	do {
>   		/*
>