Commit 200867af authored May 20, 2016 by Dan Streetman Committed by Linus Torvalds May 20, 2016

mm/zswap: use workqueue to destroy pool



Add a work_struct to struct zswap_pool, and change __zswap_pool_empty to
use the workqueue instead of using call_rcu().

When zswap destroys a pool no longer in use, it uses call_rcu() to
perform the destruction/freeing.  Since that executes in softirq
context, it must not sleep.  However, actually destroying the pool
involves freeing the per-cpu compressors (which requires locking the
cpu_add_remove_lock mutex) and freeing the zpool, for which the
implementation may sleep (e.g.  zsmalloc calls kmem_cache_destroy, which
locks the slab_mutex).  So if either mutex is currently taken, or any
other part of the compressor or zpool implementation sleeps, it will
result in a BUG().

It's not easy to reproduce this when changing zswap's params normally.
In testing with a loaded system, this does not fail:

  $ cd /sys/module/zswap/parameters
  $ echo lz4 > compressor ; echo zsmalloc > zpool

nor does this:

  $ while true ; do
  > echo lzo > compressor ; echo zbud > zpool
  > sleep 1
  > echo lz4 > compressor ; echo zsmalloc > zpool
  > sleep 1
  > done

although it's still possible either of those might fail, depending on
whether anything else besides zswap has locked the mutexes.

However, changing a parameter with no delay immediately causes the
schedule while atomic BUG:

  $ while true ; do
  > echo lzo > compressor ; echo lz4 > compressor
  > done

This is essentially the same as Yu Zhao's proposed patch to zsmalloc,
but moved to zswap, to cover compressor and zpool freeing.

Fixes: f1c54846 ("zswap: dynamic pool creation")
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Reported-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Dan Streetman <dan.streetman@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent da9556a2

mm/zswap.c

+8 −4

Original line number	Original line	Diff line number	Diff line
	@@ -117,7 +117,7 @@ struct zswap_pool {
	struct crypto_comp * __percpu *tfm;		struct crypto_comp * __percpu *tfm;
	struct kref kref;		struct kref kref;
	struct list_head list;		struct list_head list;
	struct rcu_head rcu_head;		struct work_struct work;
	struct notifier_block notifier;		struct notifier_block notifier;
	char tfm_name[CRYPTO_MAX_ALG_NAME];		char tfm_name[CRYPTO_MAX_ALG_NAME];
	};		};
	@@ -658,9 +658,11 @@ static int __must_check zswap_pool_get(struct zswap_pool *pool)
	return kref_get_unless_zero(&pool->kref);		return kref_get_unless_zero(&pool->kref);
	}		}

	static void __zswap_pool_release(struct rcu_head *head)		static void __zswap_pool_release(struct work_struct *work)
	{		{
	struct zswap_pool pool = container_of(head, typeof(pool), rcu_head);		struct zswap_pool pool = container_of(work, typeof(pool), work);

			synchronize_rcu();

	/* nobody should have been able to get a kref... */		/* nobody should have been able to get a kref... */
	WARN_ON(kref_get_unless_zero(&pool->kref));		WARN_ON(kref_get_unless_zero(&pool->kref));
	@@ -680,7 +682,9 @@ static void __zswap_pool_empty(struct kref *kref)
	WARN_ON(pool == zswap_pool_current());		WARN_ON(pool == zswap_pool_current());

	list_del_rcu(&pool->list);		list_del_rcu(&pool->list);
	call_rcu(&pool->rcu_head, __zswap_pool_release);
			INIT_WORK(&pool->work, __zswap_pool_release);
			schedule_work(&pool->work);

	spin_unlock(&zswap_pools_lock);		spin_unlock(&zswap_pools_lock);
	}		}