Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Message-ID: <52b0bb03-a6f6-b68e-49a6-e6d37fe4eaa2@linux.ibm.com>
Date:   Wed, 8 Jun 2022 13:34:26 +0530
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.10.0
Subject: Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers
Content-Language: en-US
To:     Ying Huang <ying.huang@intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, linux-mm@kvack.org,
        akpm@linux-foundation.org
Cc:     Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim C Chen <tim.c.chen@intel.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hesham Almatary <hesham.almatary@huawei.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Alistair Popple <apopple@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Feng Tang <feng.tang@intel.com>,
        Jagdish Gediya <jvgediya@linux.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        David Rientjes <rientjes@google.com>
References: <20220603134237.131362-1-aneesh.kumar@linux.ibm.com>
 <20220603134237.131362-2-aneesh.kumar@linux.ibm.com>
 <92649c9a6e0b6931b34aeaaf22c0a1e874484b7f.camel@linux.intel.com>
 <f5480a57-4335-d982-8462-d4465369082d@linux.ibm.com>
 <71a0734bc918b7a6cf75b0b411b7b6a87f0bda92.camel@intel.com>
From:   Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
In-Reply-To: <71a0734bc918b7a6cf75b0b411b7b6a87f0bda92.camel@intel.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 6/8/22 11:40 AM, Ying Huang wrote:
> On Wed, 2022-06-08 at 10:07 +0530, Aneesh Kumar K V wrote:
>> On 6/8/22 12:13 AM, Tim Chen wrote:
>> ...
>>
>>>>
>>>> +
>>>> +static void memory_tier_device_release(struct device *dev)
>>>> +{
>>>> +	struct memory_tier *tier = to_memory_tier(dev);
>>>> +
>>>
>>> Do we need some ref counts on memory_tier?
>>> If there is another device still using the same memtier,
>>> free below could cause problem.
>>>
>>>> +	kfree(tier);
>>>> +}
>>>> +
>>>>
>>> ...
>>
>> The lifecycle of the memory_tier struct is tied to the sysfs device life
>> time. ie, memory_tier_device_relese get called only after the last
>> reference on that sysfs dev object is released. Hence we can be sure
>> there is no userspace that is keeping one of the memtier related sysfs
>> file open.
>>
>> W.r.t other memory device sharing the same memtier, we unregister the
>> sysfs device only when the memory tier nodelist is empty. That is no
>> memory device is present in this memory tier.
> 
> memory_tier isn't only used by user space.  It is used inside kernel
> too.  If some kernel code get a pointer to struct memory_tier, we need
> to guarantee the pointer will not be freed under us. 

As mentioned above current patchset avoid doing that.

> And as Tim pointed
> out, we need to use it in hot path (for statistics), so some kind of rcu
> lock may be good.
> 

Sure when those statistics code get added, we can add the relevant kref 
and locking details.

-aneesh