Re: [PATCH 2/8] mm: vmscan: split unregister_shrinker()

[Dave Chinner <david@fromorbit.com>]

On Wed, May 31, 2023 at 09:57:36AM +0000, Qi Zheng wrote:
> From: Kirill Tkhai <tkhai@ya.ru>
> 
> This and the next patches in this series aim to make
> time effect of synchronize_srcu() invisible for user.
> The patch splits unregister_shrinker() in two functions:
> 
> 	unregister_shrinker_delayed_initiate()
> 	unregister_shrinker_delayed_finalize()
> 
> and shrinker users may make the second of them to be called
> asynchronous (e.g., from workqueue). Next patches make
> superblock shrinker to follow this way, so user-visible
> umount() time won't contain delays from synchronize_srcu().
> 
> Signed-off-by: Kirill Tkhai <tkhai@ya.ru>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>  include/linux/shrinker.h |  2 ++
>  mm/vmscan.c              | 22 ++++++++++++++++++----
>  2 files changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 224293b2dd06..e9d5a19d83fe 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -102,6 +102,8 @@ extern void register_shrinker_prepared(struct shrinker *shrinker);
>  extern int __printf(2, 3) register_shrinker(struct shrinker *shrinker,
>  					    const char *fmt, ...);
>  extern void unregister_shrinker(struct shrinker *shrinker);
> +extern void unregister_shrinker_delayed_initiate(struct shrinker *shrinker);
> +extern void unregister_shrinker_delayed_finalize(struct shrinker *shrinker);
>  extern void free_prealloced_shrinker(struct shrinker *shrinker);
>  extern void synchronize_shrinkers(void);
>  
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a773e97e152e..baf8d2327d70 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -799,10 +799,7 @@ int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
>  #endif
>  EXPORT_SYMBOL(register_shrinker);
>  
> -/*
> - * Remove one
> - */
> -void unregister_shrinker(struct shrinker *shrinker)
> +void unregister_shrinker_delayed_initiate(struct shrinker *shrinker)
>  {
>  	struct dentry *debugfs_entry;
>  	int debugfs_id;
> @@ -819,6 +816,13 @@ void unregister_shrinker(struct shrinker *shrinker)
>  	mutex_unlock(&shrinker_mutex);
>  
>  	shrinker_debugfs_remove(debugfs_entry, debugfs_id);
> +}
> +EXPORT_SYMBOL(unregister_shrinker_delayed_initiate);
> +
> +void unregister_shrinker_delayed_finalize(struct shrinker *shrinker)
> +{
> +	if (!shrinker->nr_deferred)
> +		return;

This is new logic and isn't explained anywhere: why do we want to
avoid RCU cleanup if (shrinker->nr_deferred == 0)? Regardless,
whatever this is avoiding, it needs a comment to explain it.

-Dave.
-- 
Dave Chinner
david@fromorbit.com