Subject: Re: [PATCH v2 03/47] mm: shrinker: add infrastructure for dynamically allocating shrinker

From: Muchun Song <[email-masked]>
Date: Tue, 25 Jul 2023 17:02:47 +0800

On 2023/7/24 17:43, 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_free_non_registered()
>
> Used to destroy the non-registered shrinker instance.

At least I don't like this name. I know you want to tell others
this function only should be called when shrinker has not been
registed but allocated. Maybe shrinker_free() is more simple.
And and a comment to tell the users when to use it.

>
> 3. shrinker_register()
>
> Used to register the shrinker instance, which is same as the current
> register_shrinker_prepared().
>
> 4. shrinker_unregister()
>
> Used to unregister 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
> setup.
> ```
>
> Signed-off-by: Qi Zheng <[email-masked]>
> ---
>   include/linux/shrinker.h |   6 +++
>   mm/shrinker.c            | 113 +++++++++++++++++++++++++++++++++++++++
>   2 files changed, 119 insertions(+)
>
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 961cb84e51f5..296f5e163861 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -70,6 +70,8 @@ struct shrinker {
>   	int seeks;	/* seeks to recreate an obj */
>   	unsigned flags;
>   
> +	void *private_data;
> +
>   	/* These are for internal use */
>   	struct list_head list;
>   #ifdef CONFIG_MEMCG
> @@ -98,6 +100,10 @@ struct shrinker {
>   
>   unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
>   			  int priority);
> +struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
> +void shrinker_free_non_registered(struct shrinker *shrinker);
> +void shrinker_register(struct shrinker *shrinker);
> +void shrinker_unregister(struct shrinker *shrinker);
>   
>   extern int __printf(2, 3) prealloc_shrinker(struct shrinker *shrinker,
>   					    const char *fmt, ...);
> diff --git a/mm/shrinker.c b/mm/shrinker.c
> index 0a32ef42f2a7..d820e4cc5806 100644
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -548,6 +548,119 @@ unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
>   	return freed;
>   }
>   
> +struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...)
> +{
> +	struct shrinker *shrinker;
> +	unsigned int size;
> +	va_list __maybe_unused ap;
> +	int err;
> +
> +	shrinker = kzalloc(sizeof(struct shrinker), GFP_KERNEL);
> +	if (!shrinker)
> +		return NULL;
> +
> +#ifdef CONFIG_SHRINKER_DEBUG
> +	va_start(ap, fmt);
> +	shrinker->name = kvasprintf_const(GFP_KERNEL, fmt, ap);
> +	va_end(ap);
> +	if (!shrinker->name)
> +		goto err_name;
> +#endif

So why not introduce another helper to handle this and declare it
as a void function when !CONFIG_SHRINKER_DEBUG? Something like the
following:

#ifdef CONFIG_SHRINKER_DEBUG
static int shrinker_debugfs_name_alloc(struct shrinker *shrinker, const 
char *fmt,
                                        va_list vargs)

{
     shrinker->name = kvasprintf_const(GFP_KERNEL, fmt, vargs);
     return shrinker->name ? 0 : -ENOMEM;
}
#else
static int shrinker_debugfs_name_alloc(struct shrinker *shrinker, const 
char *fmt,
                                        va_list vargs)
{
     return 0;
}
#endif

> +	shrinker->flags = flags;
> +
> +	if (flags & SHRINKER_MEMCG_AWARE) {
> +		err = prealloc_memcg_shrinker(shrinker);
> +		if (err == -ENOSYS)
> +			shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
> +		else if (err == 0)
> +			goto done;
> +		else
> +			goto err_flags;
> +	}
> +
> +	/*
> +	 * The nr_deferred is available on per memcg level for memcg aware
> +	 * shrinkers, so only allocate nr_deferred in the following cases:
> +	 *  - non memcg aware shrinkers
> +	 *  - !CONFIG_MEMCG
> +	 *  - memcg is disabled by kernel command line
> +	 */
> +	size = sizeof(*shrinker->nr_deferred);
> +	if (flags & SHRINKER_NUMA_AWARE)
> +		size *= nr_node_ids;
> +
> +	shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
> +	if (!shrinker->nr_deferred)
> +		goto err_flags;
> +
> +done:
> +	return shrinker;
> +
> +err_flags:
> +#ifdef CONFIG_SHRINKER_DEBUG
> +	kfree_const(shrinker->name);
> +	shrinker->name = NULL;

This could be shrinker_debugfs_name_free()

> +err_name:
> +#endif
> +	kfree(shrinker);
> +	return NULL;
> +}
> +EXPORT_SYMBOL(shrinker_alloc);
> +
> +void shrinker_free_non_registered(struct shrinker *shrinker)
> +{
> +#ifdef CONFIG_SHRINKER_DEBUG
> +	kfree_const(shrinker->name);
> +	shrinker->name = NULL;

This could be shrinker_debugfs_name_free()

> +#endif
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> +		down_write(&shrinker_rwsem);
> +		unregister_memcg_shrinker(shrinker);
> +		up_write(&shrinker_rwsem);
> +	}
> +
> +	kfree(shrinker->nr_deferred);
> +	shrinker->nr_deferred = NULL;
> +
> +	kfree(shrinker);
> +}
> +EXPORT_SYMBOL(shrinker_free_non_registered);
> +
> +void shrinker_register(struct shrinker *shrinker)
> +{
> +	down_write(&shrinker_rwsem);
> +	list_add_tail(&shrinker->list, &shrinker_list);
> +	shrinker->flags |= SHRINKER_REGISTERED;
> +	shrinker_debugfs_add(shrinker);
> +	up_write(&shrinker_rwsem);
> +}
> +EXPORT_SYMBOL(shrinker_register);
> +
> +void shrinker_unregister(struct shrinker *shrinker)

You have made all shrinkers to be dynamically allocated, so
we should prevent users from allocating shrinkers statically and
use this function to unregister it. It is better to add a
flag like SHRINKER_ALLOCATED which is set in shrinker_alloc(),
and check whether it is set in shrinker_unregister(), if not
maybe a warning should be added to tell the users what happened.

> +{
> +	struct dentry *debugfs_entry;
> +	int debugfs_id;
> +
> +	if (!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))
> +		return;
> +
> +	down_write(&shrinker_rwsem);
> +	list_del(&shrinker->list);
> +	shrinker->flags &= ~SHRINKER_REGISTERED;
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> +		unregister_memcg_shrinker(shrinker);
> +	debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);

In the internal of this function, you also could use
shrinker_debugfs_name_free().

Thanks.

> +	up_write(&shrinker_rwsem);
> +
> +	shrinker_debugfs_remove(debugfs_entry, debugfs_id);
> +
> +	kfree(shrinker->nr_deferred);
> +	shrinker->nr_deferred = NULL;
> +
> +	kfree(shrinker);
> +}
> +EXPORT_SYMBOL(shrinker_unregister);
> +
>   /*
>    * Add a shrinker callback to be called from the vm.
>    */