Re: [PATCH] mm: Use special value SHRINKER_REGISTERING instead list_empty() check

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 05.08.2018 15:50, Matthew Wilcox wrote:
> On Sun, Aug 05, 2018 at 08:30:43AM +0300, Kirill Tkhai wrote:
>> On 05.08.2018 03:03, Matthew Wilcox wrote:
>>> On Sat, Aug 04, 2018 at 09:42:05PM +0300, Kirill Tkhai wrote:
>>>> This is exactly the thing the patch makes. Instead of inserting a shrinker pointer
>>>> to idr, it inserts a fake value SHRINKER_REGISTERING there. The patch makes impossible
>>>> to dereference a shrinker unless it's completely registered. 
>>>
>>> -       id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
>>> +       id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
>>>
>>> Instead:
>>>
>>> +       id = idr_alloc(&shrinker_idr, NULL, 0, 0, GFP_KERNEL);
>>>
>>> ... and the rest of your patch becomes even simpler.
>>
>> The patch, we are discussing at the moment, does *exactly* this:
>>
>> https://lkml.org/lkml/2018/8/3/588
>>
>> It looks like you missed this hunk in the patch.
> 
> No, it does this:
> 
> +       id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
> 
> I'm saying do this:
> 
> +       id = idr_alloc(&shrinker_idr, NULL, 0, 0, GFP_KERNEL);

No, this won't work at all. The patch introduces special value SHRINKER_REGISTERING,
because shrink_slab_memcg() needs to differ the cases, when 1)shrinker is registering
and 2)shrinker is unregistered. In case of shrinker is registering we do not clear
the bit in shrink_slab_memcg(), while in the other case we must do that. This introduce
a generic solution for all type of shrinkers, and this allows to not impose restrictions
on specific shrinker registering code. A user of shrinker may add a first element to its
LRU list before register_shrinker_prepared() is called, and the corresponding bit won't
be cleared. This gives flexibility for users, it's just the same flexibility they have now.

Before the patch, list_empty() was used like such the indicator, and this is the difference
the patch makes.