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Subject[PATCH 4.4 28/87] mm/zswap: use workqueue to destroy pool
4.4-stable review patch.  If anyone has any objections, please let me know.


From: Dan Streetman <>

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 <>
Reported-by: Yu Zhao <>
Reviewed-by: Sergey Senozhatsky <>
Cc: Minchan Kim <>
Cc: Dan Streetman <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Vlastimil Babka <>
Signed-off-by: Greg Kroah-Hartman <>

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

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


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