linux-kernel - Re: [PATCH v3 2/2] virtio_balloon: replace oom notifier with shrinker

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Message-ID: <109ff5ec-692d-67fe-4c5a-2de8b48e8300@i-love.sakura.ne.jp>
Date:   Mon, 6 Aug 2018 22:28:59 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     "Wang, Wei W" <wei.w.wang@...el.com>
Cc:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 2/2] virtio_balloon: replace oom notifier with shrinker

On 2018/08/06 21:44, Wang, Wei W wrote:
> On Monday, August 6, 2018 6:29 PM, Tetsuo Handa wrote:
>> On 2018/08/06 18:56, Wei Wang wrote:
>>> On 08/03/2018 08:11 PM, Tetsuo Handa wrote:
>>>> On 2018/08/03 17:32, Wei Wang wrote:
>>>>> +static int virtio_balloon_register_shrinker(struct virtio_balloon
>>>>> +*vb) {
>>>>> +    vb->shrinker.scan_objects = virtio_balloon_shrinker_scan;
>>>>> +    vb->shrinker.count_objects = virtio_balloon_shrinker_count;
>>>>> +    vb->shrinker.batch = 0;
>>>>> +    vb->shrinker.seeks = DEFAULT_SEEKS;
>>>> Why flags field is not set? If vb is allocated by kmalloc(GFP_KERNEL)
>>>> and is nowhere zero-cleared, KASAN would complain it.
>>>
>>> Could you point where in the code that would complain it?
>>> I only see two shrinker flags (NUMA_AWARE and MEMCG_AWARE), and
>> they seem not related to that.
>>
>> Where is vb->shrinker.flags initialized?
> 
> Is that mandatory to be initialized?

Of course. ;-)

> I find it's not initialized in most shrinkers (e.g. zs_register_shrinker, huge_zero_page_shrinker).

Because most shrinkers are "statically initialized (which means that unspecified fields are
implicitly zero-cleared)" or "dynamically allocated with __GFP_ZERO or zero-cleared using
memset() (which means that all fields are once zero-cleared)".

And if you once zero-clear vb at allocation time, you will get a bonus that
calling unregister_shrinker() without corresponding register_shrinker() is safe
(which will simplify initialization failure path).