linux-kernel - Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

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Message-ID: <f5480a57-4335-d982-8462-d4465369082d@linux.ibm.com>
Date:   Wed, 8 Jun 2022 10:07:57 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

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.

-aneesh