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

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Message-ID: <52b0bb03-a6f6-b68e-49a6-e6d37fe4eaa2@linux.ibm.com>
Date:   Wed, 8 Jun 2022 13:34:26 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Ying Huang <ying.huang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Wei Xu <weixugc@...gle.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 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