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%3Ba=su...
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 stable@vger.kernel.org know about it.
From 200867af4dedfe7cb707f96773684de1d1fd21e6 Mon Sep 17 00:00:00 2001
From: Dan Streetman ddstreet@ieee.org Date: Fri, 20 May 2016 16:59:54 -0700 Subject: mm/zswap: use workqueue to destroy pool
From: Dan Streetman ddstreet@ieee.org
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
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: f1c54846ee45 ("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 Signed-off-by: Vlastimil Babka vbabka@suse.cz Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/zswap.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)
--- a/mm/zswap.c +++ b/mm/zswap.c @@ -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 ddstreet@ieee.org 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