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

[Will Deacon <will@kernel.org>]

On Wed, Jun 16, 2021 at 09:47:18AM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2021/6/15 19:55, Will Deacon wrote:
> > On Tue, Jun 15, 2021 at 12:51:38PM +0100, Robin Murphy wrote:
> >> 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.
> > 
> > So it sounds like this particular patch is fine, but the one for smmuv2
> > should leave the IRQ allocation message alone (by virtue of it printing
> > something a bit more useful -- the number of irqs).
> 
>         num_irqs = 0;
>         while ((res = platform_get_resource(pdev, IORESOURCE_IRQ, num_irqs))) {
>                 num_irqs++;
>         }
> 
> As the above code, num_irqs is calculated based on the number of dtb or acpi
> configuration items, it can't be too large. That is, there is almost zero chance
> that devm_kcalloc() will fail because num_irqs is too large.

Right, because firmware is never wrong about anything :)

Will

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