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@xxxxxxx>
> ---
> 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,
>
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