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 <stable@xxxxxxxxxxxxxxx> know about it.
>From 200867af4dedfe7cb707f96773684de1d1fd21e6 Mon Sep 17 00:00:00 2001
From: Dan Streetman <ddstreet@xxxxxxxx>
Date: Fri, 20 May 2016 16:59:54 -0700
Subject: mm/zswap: use workqueue to destroy pool
From: Dan Streetman <ddstreet@xxxxxxxx>
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@xxxxxxxx>
Reported-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Dan Streetman <dan.streetman@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxx 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