linux-ext4 - Re: [PATCH 29/29] mm: shrinker: move shrinker-related code into a separate file

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Date:   Fri, 23 Jun 2023 21:24:25 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
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,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, dm-devel@...hat.com,
        linux-raid@...r.kernel.org, linux-bcache@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 29/29] mm: shrinker: move shrinker-related code into a
 separate file

Hi Sergey,

On 2023/6/23 13:25, Sergey Senozhatsky wrote:
> On (23/06/22 16:53), Qi Zheng wrote:
>> +/*
>> + * Remove one
>> + */
>> +void unregister_shrinker(struct shrinker *shrinker)
>> +{
>> +	struct dentry *debugfs_entry;
>> +	int debugfs_id;
>> +
>> +	if (!(shrinker->flags & SHRINKER_REGISTERED))
>> +		return;
>> +
>> +	shrinker_put(shrinker);
>> +	wait_for_completion(&shrinker->completion_wait);
>> +
>> +	mutex_lock(&shrinker_mutex);
>> +	list_del_rcu(&shrinker->list);
> 
> Should this function wait for RCU grace period(s) before it goes
> touching shrinker fields?

Why? We will free this shrinker instance by rcu after executing
unregister_shrinker(). So it is safe to touch shrinker fields here.

> 
>> +	shrinker->flags &= ~SHRINKER_REGISTERED;
>> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>> +		unregister_memcg_shrinker(shrinker);
>> +	debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
>> +	mutex_unlock(&shrinker_mutex);
>> +
>> +	shrinker_debugfs_remove(debugfs_entry, debugfs_id);
>> +
>> +	kfree(shrinker->nr_deferred);
>> +	shrinker->nr_deferred = NULL;
>> +}
>> +EXPORT_SYMBOL(unregister_shrinker);
> 
> [..]
> 
>> +void shrinker_free(struct shrinker *shrinker)
>> +{
>> +	kfree(shrinker);
>> +}
>> +EXPORT_SYMBOL(shrinker_free);
>> +
>> +void unregister_and_free_shrinker(struct shrinker *shrinker)
>> +{
>> +	unregister_shrinker(shrinker);
>> +	kfree_rcu(shrinker, rcu);
>> +}
> 
> Seems like this
> 
> 	unregister_shrinker();
> 	shrinker_free();
> 
> is not exact equivalent of this
> 
> 	unregister_and_free_shrinker();

Yes, my original intention is that shrinker_free() is only used to
handle the case where register_shrinker() returns failure.

I will implement the method suggested by Dave in 02/29. Those APIs are
more concise and will bring more benefits. :)

Thanks,
Qi