zswap: don't param_set_charp while holding spinlock

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")
Link: http://lkml.kernel.org/r/20170126155821.4545-1-ddstreet@ieee.org
Signed-off-by: Dan Streetman <dan.streetman@canonical.com>
Reported-by: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 7 deletions

diff --git a/mm/zswap.c b/mm/zswap.c
index 9e8565d9c66b..eedc27894b10 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
          */