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

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

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) {
>>>>> +A A A  vb->shrinker.scan_objects = virtio_balloon_shrinker_scan;
>>>>> +A A A  vb->shrinker.count_objects = virtio_balloon_shrinker_count;
>>>>> +A A A  vb->shrinker.batch = 0;
>>>>> +A A A  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).