Patch "mm/zswap: use workqueue to destroy pool" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

mm/zswap: use workqueue to destroy pool

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-zswap-use-workqueue-to-destroy-pool.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dan Streetman <[email protected]>
Date: Fri, 20 May 2016 16:59:54 -0700
Subject: mm/zswap: use workqueue to destroy pool

From: Dan Streetman <[email protected]>

commit 200867af4dedfe7cb707f96773684de1d1fd21e6 upstream.

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

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

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

---
 mm/zswap.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

@@ -123,7 +123,7 @@ struct zswap_pool {
 	struct crypto_comp * __percpu *tfm;
 	struct kref kref;
 	struct list_head list;
-	struct rcu_head rcu_head;
+	struct work_struct work;
 	struct notifier_block notifier;
 	char tfm_name[CRYPTO_MAX_ALG_NAME];
 };

@@ -667,9 +667,11 @@ static int __must_check zswap_pool_get(s
 	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... */
 	WARN_ON(kref_get_unless_zero(&pool->kref));

@@ -689,7 +691,9 @@ static void __zswap_pool_empty(struct kr
 	WARN_ON(pool == zswap_pool_current());
 
 	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);
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mm-zswap-use-workqueue-to-destroy-pool.patch queue-4.4/zswap-don-t-param_set_charp-while-holding-spinlock.patch