The patch titled
Subject: mm/zswap: use workqueue to destroy pool
has been added to the -mm tree. Its filename is
mm-zswap-use-workqueue-to-destroy-pool.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-zswap-use-workqueue-to-destroy-pool.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-zswap-use-workqueue-to-destroy-pool.patch
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From: Dan Streetman <ddstreet@xxxxxxxx>
Subject: 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: f1c54846ee45 ("zswap: dynamic pool creation")
Signed-off-by: Dan Streetman <ddstreet@xxxxxxxx>
Reported-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Dan Streetman <dan.streetman@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/zswap.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff -puN mm/zswap.c~mm-zswap-use-workqueue-to-destroy-pool mm/zswap.c
--- a/mm/zswap.c~mm-zswap-use-workqueue-to-destroy-pool
+++ a/mm/zswap.c
@@ -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];
};
@@ -652,9 +652,11 @@ 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));
@@ -674,7 +676,9 @@ 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 -mm which might be from ddstreet@xxxxxxxx are
mm-zpool-use-workqueue-for-zpool_destroy.patch
mm-zswap-use-workqueue-to-destroy-pool.patch
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