Re: [PATCH 1/1] iommu/arm-smmu-v3: remove unnecessary oom message

[Robin Murphy <robin.murphy@arm.com>]

On 2021-06-15 12:34, Will Deacon wrote:
> On Tue, Jun 15, 2021 at 07:22:10PM +0800, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2021/6/11 18:32, Will Deacon wrote:
>>> On Wed, Jun 09, 2021 at 08:54:38PM +0800, Zhen Lei wrote:
>>>> Fixes scripts/checkpatch.pl warning:
>>>> WARNING: Possible unnecessary 'out of memory' message
>>>>
>>>> Remove it can help us save a bit of memory.
>>>>
>>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>>>> ---
>>>>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 8 ++------
>>>>   1 file changed, 2 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>>> index 2ddc3cd5a7d1..fd7c55b44881 100644
>>>> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>>> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>>> @@ -2787,10 +2787,8 @@ static int arm_smmu_init_l1_strtab(struct arm_smmu_device *smmu)
>>>>   	void *strtab = smmu->strtab_cfg.strtab;
>>>>   
>>>>   	cfg->l1_desc = devm_kzalloc(smmu->dev, size, GFP_KERNEL);
>>>> -	if (!cfg->l1_desc) {
>>>> -		dev_err(smmu->dev, "failed to allocate l1 stream table desc\n");
>>>> +	if (!cfg->l1_desc)
>>>
>>> What error do you get if devm_kzalloc() fails? I'd like to make sure it's
>>> easy to track down _which_ allocation failed in that case -- does it give
>>> you a line number, for example?
>>
>> When devm_kzalloc() fails, the OOM information is printed. No line number information, but the
>> size(order) and call stack is printed. It doesn't matter which allocation failed, the failure
>> is caused by insufficient system memory rather than the fault of the SMMU driver. Therefore,
>> the current printing is not helpful for locating the problem of insufficient memory. After all,
>> when memory allocation fails, the SMMU driver cannot work at a lower specification.
> 
> I don't entirely agree. Another reason for the failure is because the driver
> might be asking for a huge (or negative) allocation, in which case it might
> be instructive to have a look at the actual caller, particularly if the
> size is derived from hardware or firmware properties.

Agreed - other than deliberately-contrived situations I don't think I've 
ever hit a genuine OOM, but I definitely have debugged attempts to 
allocate -1 of something. If the driver-specific message actually calls 
out the critical information, e.g. "failed to allocate %d stream table 
entries", it gives debugging a head start since the miscalculation is 
obvious, but a static message that only identifies the callsite really 
only saves a quick trip to scripts/faddr2line, and personally I've never 
found that particularly valuable.

Robin.

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