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>

diff --git a/mm/zswap.c b/mm/zswap.c
index 9e8565d9c66beebc14c93dcf8e03a7f76b73f334..eedc27894b1013187cc81831e956e4c8344b5176 100644 (file)
--- 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
         */