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Date:   Tue, 9 Feb 2021 16:39:43 -0800
From:   Roman Gushchin <guro@fb.com>
To:     Yang Shi <shy828301@gmail.com>
CC:     <ktkhai@virtuozzo.com>, <vbabka@suse.cz>, <shakeelb@google.com>,
        <david@fromorbit.com>, <hannes@cmpxchg.org>, <mhocko@suse.com>,
        <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [v7 PATCH 07/12] mm: vmscan: use a new flag to indicate shrinker
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On Tue, Feb 09, 2021 at 09:46:41AM -0800, Yang Shi wrote:
> Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
> This approach is fine with nr_deferred at the shrinker level, but the following
> patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their
> shrinker->nr_deferred would always be NULL.  This would prevent the shrinkers
> from unregistering correctly.
> 
> Remove SHRINKER_REGISTERING since we could check if shrinker is registered
> successfully by the new flag.
> 
> Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  include/linux/shrinker.h |  7 ++++---
>  mm/vmscan.c              | 31 +++++++++----------------------
>  2 files changed, 13 insertions(+), 25 deletions(-)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 0f80123650e2..1eac79ce57d4 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -79,13 +79,14 @@ struct shrinker {
>  #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
>  
>  /* Flags */
> -#define SHRINKER_NUMA_AWARE	(1 << 0)
> -#define SHRINKER_MEMCG_AWARE	(1 << 1)
> +#define SHRINKER_REGISTERED	(1 << 0)
> +#define SHRINKER_NUMA_AWARE	(1 << 1)
> +#define SHRINKER_MEMCG_AWARE	(1 << 2)
>  /*
>   * It just makes sense when the shrinker is also MEMCG_AWARE for now,
>   * non-MEMCG_AWARE shrinker should not have this flag set.
>   */
> -#define SHRINKER_NONSLAB	(1 << 2)
> +#define SHRINKER_NONSLAB	(1 << 3)
>  
>  extern int prealloc_shrinker(struct shrinker *shrinker);
>  extern void register_shrinker_prepared(struct shrinker *shrinker);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 273efbf4d53c..a047980536cf 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -315,19 +315,6 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
>  	}
>  }
>  
> -/*
> - * We allow subsystems to populate their shrinker-related
> - * LRU lists before register_shrinker_prepared() is called
> - * for the shrinker, since we don't want to impose
> - * restrictions on their internal registration order.
> - * In this case shrink_slab_memcg() may find corresponding
> - * bit is set in the shrinkers map.
> - *
> - * This value is used by the function to detect registering
> - * shrinkers and to skip do_shrink_slab() calls for them.
> - */
> -#define SHRINKER_REGISTERING ((struct shrinker *)~0UL)
> -
>  static DEFINE_IDR(shrinker_idr);
>  
>  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> @@ -336,7 +323,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>  
>  	down_write(&shrinker_rwsem);
>  	/* This may call shrinker, so it must use down_read_trylock() */
> -	id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
> +	id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
>  	if (id < 0)
>  		goto unlock;
>  
> @@ -499,10 +486,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
>  {
>  	down_write(&shrinker_rwsem);
>  	list_add_tail(&shrinker->list, &shrinker_list);
> -#ifdef CONFIG_MEMCG
> -	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> -		idr_replace(&shrinker_idr, shrinker, shrinker->id);
> -#endif
> +	shrinker->flags |= SHRINKER_REGISTERED;
>  	up_write(&shrinker_rwsem);
>  }
>  
> @@ -522,13 +506,16 @@ EXPORT_SYMBOL(register_shrinker);
>   */
>  void unregister_shrinker(struct shrinker *shrinker)
>  {
> -	if (!shrinker->nr_deferred)
> +	if (!(shrinker->flags & SHRINKER_REGISTERED))
>  		return;
> -	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> -		unregister_memcg_shrinker(shrinker);
> +
>  	down_write(&shrinker_rwsem);
>  	list_del(&shrinker->list);
> +	shrinker->flags &= ~SHRINKER_REGISTERED;
>  	up_write(&shrinker_rwsem);
> +
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> +		unregister_memcg_shrinker(shrinker);

Because unregister_memcg_shrinker() will take and release shrinker_rwsem once again,
I wonder if it's better to move it into the locked section and change the calling
convention to require the caller to take the semaphore?

>  	kfree(shrinkrem->nr_deferred);
>  	shrinker->nr_deferred = NULL;
>  }
> @@ -693,7 +680,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>  		struct shrinker *shrinker;
>  
>  		shrinker = idr_find(&shrinker_idr, i);
> -		if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) {
> +		if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) {
>  			if (!shrinker)
>  				clear_bit(i, info->map);
>  			continue;
> -- 
> 2.26.2
>