Commit 200867af authored by Dan Streetman's avatar Dan Streetman Committed by Linus Torvalds

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: default avatarDan Streetman <>
Reported-by: default avatarYu Zhao <>
Reviewed-by: default avatarSergey Senozhatsky <>
Cc: Minchan Kim <>
Cc: Dan Streetman <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent da9556a2
......@@ -117,7 +117,7 @@ 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];
......@@ -658,9 +658,11 @@ static int __must_check zswap_pool_get(struct zswap_pool *pool)
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);
/* nobody should have been able to get a kref... */
......@@ -680,7 +682,9 @@ static void __zswap_pool_empty(struct kref *kref)
WARN_ON(pool == zswap_pool_current());
call_rcu(&pool->rcu_head, __zswap_pool_release);
INIT_WORK(&pool->work, __zswap_pool_release);
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