linux-kernel - Re: [PATCH v4 06/13] KVM: Move WARN on invalid memslot index to update

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Message-ID: <fa71d652-8b7f-e0d7-5617-8958e3e78f6e@maciej.szmigiero.name>
Date:   Wed, 18 Aug 2021 23:43:21 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     David Hildenbrand <david@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v4 06/13] KVM: Move WARN on invalid memslot index to
 update_memslots()

On 18.08.2021 16:35, David Hildenbrand wrote:
> On 13.08.21 21:33, Maciej S. Szmigiero wrote:
>> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>>
>> Since kvm_memslot_move_forward() can theoretically return a negative
>> memslot index even when kvm_memslot_move_backward() returned a positive one
>> (and so did not WARN) let's just move the warning to the common code.
>>
>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
>> ---
>>   virt/kvm/kvm_main.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 03ef42d2e421..7000efff1425 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -1293,8 +1293,7 @@ static inline int kvm_memslot_move_backward(struct kvm_memslots *slots,
>>       struct kvm_memory_slot *mslots = slots->memslots;
>>       int i;
>> -    if (WARN_ON_ONCE(slots->id_to_index[memslot->id] == -1) ||
>> -        WARN_ON_ONCE(!slots->used_slots))
>> +    if (slots->id_to_index[memslot->id] == -1 || !slots->used_slots)
>>           return -1;
>>       /*
>> @@ -1398,6 +1397,9 @@ static void update_memslots(struct kvm_memslots *slots,
>>               i = kvm_memslot_move_backward(slots, memslot);
>>           i = kvm_memslot_move_forward(slots, memslot, i);
>> +        if (WARN_ON_ONCE(i < 0))
>> +            return;
>> +
>>           /*
>>            * Copy the memslot to its new position in memslots and update
>>            * its index accordingly.
>>
> 
> 
> Note that WARN_ON_* is frowned upon, because it can result in crashes with panic_on_warn enabled, which is what some distributions do enable.
> 
> We tend to work around that by using pr_warn()/pr_warn_once(), avoiding eventually crashing the system when there is a way to continue.
> 

This patch uses WARN_ON_ONCE because:
1) It was used in the old code and the patch merely moves the check
from kvm_memslot_move_backward() to its caller,

2) This chunk of code is wholly replaced by patch 11 from this series
anyway ("Keep memslots in tree-based structures instead of array-based ones").

Thanks,
Maciej