linux-kernel - Re: [PATCH 1/2] mm/vmscan: add sync

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Date:   Fri, 9 Apr 2021 13:00:22 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christian König <ckoenig.leichtzumerken@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     ray.huang@....com, daniel@...ll.ch, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/2] mm/vmscan: add sync_shrinkers function

On 4/9/21 9:17 AM, Christian König wrote:
> To be able to switch to a spinlock and reduce lock contention in the TTM
> shrinker we don't want to hold a mutex while unmapping and freeing pages
> from the pool.

Does using spinlock instead of mutex really reduce lock contention?

> But then we somehow need to prevent a race between (for example) the shrinker
> trying to free pages and hotplug trying to remove the device which those pages
> belong to.
> 
> Taking and releasing the shrinker semaphore on the write side after
> unmapping and freeing all pages should make sure that no shrinker is running in
> paralell any more.

So you explain this in this commit log for adding the function, but then the
next patch just adds a sync_shrinkers() call without any comment. I would expect
there a comment explaining why it's done there - what it protects against, as
it's not an obvious pattern IMHO.

> Signed-off-by: Christian König <christian.koenig@....com>
> ---
>  include/linux/shrinker.h |  1 +
>  mm/vmscan.c              | 10 ++++++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 0f80123650e2..6b75dc372fce 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -92,4 +92,5 @@ extern void register_shrinker_prepared(struct shrinker *shrinker);
>  extern int register_shrinker(struct shrinker *shrinker);
>  extern void unregister_shrinker(struct shrinker *shrinker);
>  extern void free_prealloced_shrinker(struct shrinker *shrinker);
> +extern void sync_shrinkers(void);
>  #endif
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 562e87cbd7a1..46cd9c215d73 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -408,6 +408,16 @@ void unregister_shrinker(struct shrinker *shrinker)
>  }
>  EXPORT_SYMBOL(unregister_shrinker);
>  
> +/**
> + * sync_shrinker - Wait for all running shrinkers to complete.
> + */
> +void sync_shrinkers(void)
> +{
> +	down_write(&shrinker_rwsem);
> +	up_write(&shrinker_rwsem);
> +}
> +EXPORT_SYMBOL(sync_shrinkers);
> +
>  #define SHRINK_BATCH 128
>  
>  static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>