[PATCH] zswap: don't param_set_charp while holding spinlock

[Dan Streetman <ddstreet@ieee.org>]

Change the zpool/compressor param callback function to release the
zswap_pools_lock spinlock before calling param_set_charp, since
that function may sleep when it calls kmalloc with GFP_KERNEL.

While this problem has existed for a while, I wasn't able to trigger
it using a tight loop changing either/both the zpool and compressor
params; I think it's very unlikely to be an issue on the stable kernels,
especially since most zswap users will change the compressor and/or
zpool from sysfs only one time each boot - or zero times, if they add
the params to the kernel boot.

Fixes: c99b42c3529e ("zswap: use charp for zswap param strings")
Reported-by: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Dan Streetman <dan.streetman@canonical.com>
---
Andrew, I'll leave it up to you if you want to send this to -stable;
personally I don't think it's needed.  For the stable kernels, only
the first hunk of the patch applies.

 mm/zswap.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 9e8565d..eedc278 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -704,18 +704,22 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 	pool = zswap_pool_find_get(type, compressor);
 	if (pool) {
 		zswap_pool_debug("using existing", pool);
+		WARN_ON(pool == zswap_pool_current());
 		list_del_rcu(&pool->list);
-	} else {
-		spin_unlock(&zswap_pools_lock);
-		pool = zswap_pool_create(type, compressor);
-		spin_lock(&zswap_pools_lock);
 	}
 
+	spin_unlock(&zswap_pools_lock);
+
+	if (!pool)
+		pool = zswap_pool_create(type, compressor);
+
 	if (pool)
 		ret = param_set_charp(s, kp);
 	else
 		ret = -EINVAL;
 
+	spin_lock(&zswap_pools_lock);
+
 	if (!ret) {
 		put_pool = zswap_pool_current();
 		list_add_rcu(&pool->list, &zswap_pools);
@@ -727,7 +731,11 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 		 */
 		list_add_tail_rcu(&pool->list, &zswap_pools);
 		put_pool = pool;
-	} else if (!zswap_has_pool) {
+	}
+
+	spin_unlock(&zswap_pools_lock);
+
+	if (!zswap_has_pool && !pool) {
 		/* if initial pool creation failed, and this pool creation also
 		 * failed, maybe both compressor and zpool params were bad.
 		 * Allow changing this param, so pool creation will succeed
@@ -738,8 +746,6 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 		ret = param_set_charp(s, kp);
 	}
 
-	spin_unlock(&zswap_pools_lock);
-
 	/* drop the ref from either the old current pool,
 	 * or the new pool we failed to add
 	 */
-- 
2.9.3

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