linux-ext4 - Re: [PATCH v3 05/49] mm: shrinker: add infrastructure for dynamically allocating shrinker

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Date:   Fri, 28 Jul 2023 14:17:23 +0200
From:   Simon Horman <simon.horman@...igine.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     akpm@...ux-foundation.org, david@...morbit.com, tkhai@...ru,
        vbabka@...e.cz, roman.gushchin@...ux.dev, djwong@...nel.org,
        brauner@...nel.org, paulmck@...nel.org, tytso@....edu,
        steven.price@....com, cel@...nel.org, senozhatsky@...omium.org,
        yujie.liu@...el.com, gregkh@...uxfoundation.org,
        muchun.song@...ux.dev, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-erofs@...ts.ozlabs.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
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        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v3 05/49] mm: shrinker: add infrastructure for
 dynamically allocating shrinker

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

> 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?

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

...