netdev - Re: [PATCH v3 05/49] mm: shrinker: add infrastructure for dynamically allocating shrinker

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Message-ID: <5e50711c-a616-f95f-d6d2-c69627ac3cf0@bytedance.com>
Date: Sat, 29 Jul 2023 16:47:57 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Simon Horman <simon.horman@...igine.com>
Cc: akpm@...ux-foundation.org, david@...morbit.com, tkhai@...ru,
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Subject: Re: [PATCH v3 05/49] mm: shrinker: add infrastructure for dynamically
 allocating shrinker

Hi Simon,

On 2023/7/28 20:17, Simon Horman wrote:
> On Thu, Jul 27, 2023 at 04:04:18PM +0800, Qi Zheng wrote:
>> Currently, the shrinker instances can be divided into the following three
>> types:
>>
>> a) global shrinker instance statically defined in the kernel, such as
>>     workingset_shadow_shrinker.
>>
>> b) global shrinker instance statically defined in the kernel modules, such
>>     as mmu_shrinker in x86.
>>
>> c) shrinker instance embedded in other structures.
>>
>> For case a, the memory of shrinker instance is never freed. For case b,
>> the memory of shrinker instance will be freed after synchronize_rcu() when
>> the module is unloaded. For case c, the memory of shrinker instance will
>> be freed along with the structure it is embedded in.
>>
>> In preparation for implementing lockless slab shrink, we need to
>> dynamically allocate those shrinker instances in case c, then the memory
>> can be dynamically freed alone by calling kfree_rcu().
>>
>> So this commit adds the following new APIs for dynamically allocating
>> shrinker, and add a private_data field to struct shrinker to record and
>> get the original embedded structure.
>>
>> 1. shrinker_alloc()
>>
>> Used to allocate shrinker instance itself and related memory, it will
>> return a pointer to the shrinker instance on success and NULL on failure.
>>
>> 2. shrinker_register()
>>
>> Used to register the shrinker instance, which is same as the current
>> register_shrinker_prepared().
>>
>> 3. shrinker_free()
>>
>> Used to unregister (if needed) and free the shrinker instance.
>>
>> In order to simplify shrinker-related APIs and make shrinker more
>> independent of other kernel mechanisms, subsequent submissions will use
>> the above API to convert all shrinkers (including case a and b) to
>> dynamically allocated, and then remove all existing APIs.
>>
>> This will also have another advantage mentioned by Dave Chinner:
>>
>> ```
>> The other advantage of this is that it will break all the existing
>> out of tree code and third party modules using the old API and will
>> no longer work with a kernel using lockless slab shrinkers. They
>> need to break (both at the source and binary levels) to stop bad
>> things from happening due to using uncoverted shrinkers in the new
> 
> nit: uncoverted -> unconverted

Thanks. Will fix.

> 
>> setup.
>> ```
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> 
> ...
> 
>> diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
>> index f1becfd45853..506257585408 100644
>> --- a/mm/shrinker_debug.c
>> +++ b/mm/shrinker_debug.c
>> @@ -191,6 +191,20 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
>>   	return 0;
>>   }
>>   
>> +int shrinker_debugfs_name_alloc(struct shrinker *shrinker, const char *fmt,
>> +				va_list ap)
>> +{
>> +	shrinker->name = kvasprintf_const(GFP_KERNEL, fmt, ap);
>> +
>> +	return shrinker->name ? 0 : -ENOMEM;
>> +}
>> +
>> +void shrinker_debugfs_name_free(struct shrinker *shrinker)
>> +{
>> +	kfree_const(shrinker->name);
>> +	shrinker->name = NULL;
>> +}
>> +
> 
> These functions have no prototype in this file,
> perhaps internal.h should be included?

The compiler can find these implementations, so I don't think there
is a need to include internal.h here?

Thanks,
Qi

> 
>>   int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
>>   {
>>   	struct dentry *entry;
> 
> ...