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

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Message-ID: <YHg+X7KLCmn2+yqw@phenom.ffwll.local>
Date:   Thu, 15 Apr 2021 15:23:43 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, vbabka@...e.cz, daniel@...ll.ch,
        ray.huang@....com, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/2] mm/vmscan: add sync_shrinkers function

On Thu, Apr 15, 2021 at 01:56:23PM +0200, 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.
> 
> 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.
> 
> 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.

Maybe make it clear this is a barrier type thing, it wont stop shrinkers
at all, just synchronize with them.

Acked-by: Daniel Vetter <daniel.vetter@...ll.ch>

But needs an ack from Andrew for merging through drm-misc-next before we
push it there.
-Daniel

> + */
> +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,
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch