linux-kernel - Re: [PATCH 1/4] KVM-S390: Improve determination of sizes in kvm_s390_import_bp

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Date:	Thu, 18 Aug 2016 12:52:11 +0200
From:	walter harms <wharms@....de>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	Julia Lawall <julia.lawall@...6.fr>,
	SF Markus Elfring <elfring@...rs.sourceforge.net>,
	kvm@...r.kernel.org, linux-s390@...r.kernel.org,
	Christian Bornträger 
	<borntraeger@...ibm.com>, Cornelia Huck <cornelia.huck@...ibm.com>,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Radim Krčmář 
	<rkrcmar@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM-S390: Improve determination of sizes in kvm_s390_import_bp_data()



Am 18.08.2016 11:48, schrieb Paolo Bonzini:
> 
> 
> On 18/08/2016 11:02, Julia Lawall wrote:
>>
>>
>> On Thu, 18 Aug 2016, walter harms wrote:
>>
>>>
>>>
>>> Am 17.08.2016 20:06, schrieb SF Markus Elfring:
>>>> From: Markus Elfring <elfring@...rs.sourceforge.net>
>>>> Date: Wed, 17 Aug 2016 18:29:04 +0200
>>>>
>>>> Replace the specification of data structures by pointer dereferences
>>>> to make the corresponding size determination a bit safer according to
>>>> the Linux coding style convention.
>>>>
>>>> Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
>>>> ---
>>>>  arch/s390/kvm/guestdbg.c | 6 +++---
>>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/arch/s390/kvm/guestdbg.c b/arch/s390/kvm/guestdbg.c
>>>> index d1f8241..b68db4b 100644
>>>> --- a/arch/s390/kvm/guestdbg.c
>>>> +++ b/arch/s390/kvm/guestdbg.c
>>>> @@ -216,7 +216,7 @@ int kvm_s390_import_bp_data(struct kvm_vcpu *vcpu,
>>>>  	else if (dbg->arch.nr_hw_bp > MAX_BP_COUNT)
>>>>  		return -EINVAL;
>>>>
>>>> -	size = dbg->arch.nr_hw_bp * sizeof(struct kvm_hw_breakpoint);
>>>> +	size = dbg->arch.nr_hw_bp * sizeof(*bp_data);
>>>>  	bp_data = kmalloc(size, GFP_KERNEL);
>>>>  	if (!bp_data) {
>>>>  		ret = -ENOMEM;
>>>> @@ -241,7 +241,7 @@ int kvm_s390_import_bp_data(struct kvm_vcpu *vcpu,
>>>>  		}
>>>>  	}
>>>>
>>>> -	size = nr_wp * sizeof(struct kvm_hw_wp_info_arch);
>>>> +	size = nr_wp * sizeof(*wp_info);
>>>>  	if (size > 0) {
>>>>  		wp_info = kmalloc(size, GFP_KERNEL);
>>>>  		if (!wp_info) {
>>>> @@ -249,7 +249,7 @@ int kvm_s390_import_bp_data(struct kvm_vcpu *vcpu,
>>>>  			goto error;
>>>>  		}
>>>>  	}
>>>> -	size = nr_bp * sizeof(struct kvm_hw_bp_info_arch);
>>>> +	size = nr_bp * sizeof(*bp_info);
>>>>  	if (size > 0) {
>>>>  		bp_info = kmalloc(size, GFP_KERNEL);
>>>>  		if (!bp_info) {
>>>
>>>
>>> IMHO the common pattern for kmalloc is
>>>   bp_info = kmalloc( nr_bp * sizeof(*bp_info), GFP_KERNEL);
>>>
>>> i can not remember code with a check for size < 0, i guess it is here
>>> to avoid an overflow ? since kmalloc takes a size_t argument this would cause
>>> a malloc failure an can be ignored.
>>
>> Shoudn't it be kcalloc?
> 
> Or kmalloc_array, since zeroing is not necessary.  Might be an idea for
> a new Coccinelle script, like
> 
> - kmalloc (N * sizeof T, GFP)
> + kmalloc_array(N, sizeof T, GFP)
> 


my personal taste is to stay close to the libc functions.
technical there is no difference

static inline void *kcalloc(size_t n, size_t size, gfp_t flags)
 {
        return kmalloc_array(n, size, flags | __GFP_ZERO);
 }

and i do not see any time critical things here,


re,
 wh




> Thanks,
> 
> Paolo
> 
>>
>> julia
>>
>>>
>>>
>>> just my 2 cents.
>>> re,
>>>  wh
>>>
>>
>