linux-kernel - Re: [PATCH v5 04/21] x86/resctrl: Group struct rdt_hw

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Date:   Wed, 29 Jun 2022 12:07:20 +0100
From:   James Morse <james.morse@....com>
To:     "tan.shaopeng@...itsu.com" <tan.shaopeng@...itsu.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        "lcherian@...vell.com" <lcherian@...vell.com>,
        "bobo.shaobowang@...wei.com" <bobo.shaobowang@...wei.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Cristian Marussi <cristian.marussi@....com>,
        Xin Hao <xhao@...ux.alibaba.com>,
        "xingxin.hx@...nanolis.org" <xingxin.hx@...nanolis.org>,
        "baolin.wang@...ux.alibaba.com" <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v5 04/21] x86/resctrl: Group struct rdt_hw_domain cleanup

Hi Shaopeng,

On 29/06/2022 09:33, tan.shaopeng@...itsu.com wrote:
>> domain_add_cpu() and domain_remove_cpu() need to kfree() the child arrays
>> that were allocated by domain_setup_ctrlval().
>>
>> As this memory is moved around, and new arrays are created, adjusting the
>> error handling cleanup code becomes noisier.
>>
>> To simplify this, move all the kfree() calls into a domain_free() helper.
>> This depends on struct rdt_hw_domain being kzalloc()d, allowing it to
>> unconditionally kfree() all the child arrays.

>> diff --git a/arch/x86/kernel/cpu/resctrl/core.c
>> b/arch/x86/kernel/cpu/resctrl/core.c
>> index 25f30148478b..e37889f7a1a5 100644
>> --- a/arch/x86/kernel/cpu/resctrl/core.c
>> +++ b/arch/x86/kernel/cpu/resctrl/core.c
>> @@ -414,6 +414,13 @@ void setup_default_ctrlval(struct rdt_resource *r, u32
>> *dc, u32 *dm)
>>  	}
>>  }
>>
>> +static void domain_free(struct rdt_hw_domain *hw_dom) {
>> +	kfree(hw_dom->ctrl_val);
>> +	kfree(hw_dom->mbps_val);
>> +	kfree(hw_dom);
>> +}
>> +
>>  static int domain_setup_ctrlval(struct rdt_resource *r, struct rdt_domain *d)  {
>>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r); @@ -488,7
>> +495,7 @@ static void domain_add_cpu(int cpu, struct rdt_resource *r)
>>  	rdt_domain_reconfigure_cdp(r);
>>
>>  	if (r->alloc_capable && domain_setup_ctrlval(r, d)) {
>> -		kfree(hw_dom);
>> +		domain_free(hw_dom);

> domain_free(hw_dom) is executed when fails allocated hw_dom->ctrl_val 
> by kmalloc_array() in domain_setup_ctrlval(r, d), 
> but hw_dom->ctrl_val is freed in domain_free(hw_dom).
> 
> Also, hw_dom->mbps_val is not allocated at this time,
> but it is freed in domain_free(hw_dom).

Yes, this is deliberate. These cases end up doing:
|	kfree(NULL);
which is harmless. kfree() checks for a NULL argument and does nothing.

The alternative would be to spread the cleanup all over the place, so it only calls
kfree() on something that has been allocated - this would be more complex and easier to
miss something.


> In addition，I tested this patch series on Intel(R) Xeon(R) Gold 6254 CPU with resctrl selftest.
> It is no problem.

Thanks!

James