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

From: Dave Chinner <[email-masked]>
Date: Wed, 26 Jul 2023 17:26:04 +1000

On Mon, Jul 24, 2023 at 05:43:10PM +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_free_non_registered()
> 
> Used to destroy the non-registered shrinker instance.

This is a bit nasty

> 
> 3. shrinker_register()
> 
> Used to register the shrinker instance, which is same as the current
> register_shrinker_prepared().
> 
> 4. shrinker_unregister()

rename this "shrinker_free()" and key the two different freeing
cases on the SHRINKER_REGISTERED bit rather than mostly duplicating
the two.

void shrinker_free(struct shrinker *shrinker)
{
	struct dentry *debugfs_entry = NULL;
	int debugfs_id;

	if (!shrinker)
		return;

	down_write(&shrinker_rwsem);
	if (shrinker->flags & SHRINKER_REGISTERED) {
		list_del(&shrinker->list);
		debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
	} else if (IS_ENABLED(CONFIG_SHRINKER_DEBUG)) {
		kfree_const(shrinker->name);
	}

	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
		unregister_memcg_shrinker(shrinker);
	up_write(&shrinker_rwsem);

	if (debugfs_entry)
		shrinker_debugfs_remove(debugfs_entry, debugfs_id);

	kfree(shrinker->nr_deferred);
	kfree(shrinker);
}
EXPORT_SYMBOL_GPL(shrinker_free);

-- 
Dave Chinner
[email-masked]