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 12:55:11 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	wharms@....de
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()



On 18/08/2016 12:52, walter harms wrote:
> 
> 
> 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,

This is _not_ premature optimization.  (k)calloc tells the reader that
it's safe not to initialize part of the array.  kmalloc_array says the
opposite.  Using the right function adds important hints in the
code---which unlike comments cannot get stale without also introducing
visible bugs.

Paolo