Re: [PATCH 1/4] KVM-S390: Improve determination of sizes in kvm_s390_import_bp_data()

Paolo Bonzini
2016-08-18T10:55:11Z

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@users.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@users.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