zswap: do not allocate from atomic pool

zswap_frontswap_load() should be called from preemptible context (we even
call mutex_lock() there) and it does not look like we need to do
GFP_ATOMIC allocaion for temp buffer.  The same applies to
zswap_writeback_entry().

Use GFP_KERNEL for temporary buffer allocation in both cases.

Link: https://lkml.kernel.org/r/Y3xCTr6ikbtcUr/y@google.com
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zpool.c

@@ -387,6 +387,13 @@ bool zpool_evictable(struct zpool *zpool)
  * zpool_can_sleep_mapped - Test if zpool can sleep when do mapped.
  * @zpool:	The zpool to test
  *
+ * Some allocators enter non-preemptible context in ->map() callback (e.g.
+ * disable pagefaults) and exit that context in ->unmap(), which limits what
+ * we can do with the mapped object. For instance, we cannot wait for
+ * asynchronous crypto API to decompress such an object or take mutexes
+ * since those will call into the scheduler. This function tells us whether
+ * we use such an allocator.
+ *
  * Returns: true if zpool can sleep; false otherwise.
  */
 bool zpool_can_sleep_mapped(struct zpool *zpool)

mm/zswap.c

@@ -958,7 +958,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 	};
 
 	if (!zpool_can_sleep_mapped(pool)) {
-		tmp = kmalloc(PAGE_SIZE, GFP_ATOMIC);
+		tmp = kmalloc(PAGE_SIZE, GFP_KERNEL);
 		if (!tmp)
 			return -ENOMEM;
 	}
@@ -1311,7 +1311,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 	}
 
 	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
-		tmp = kmalloc(entry->length, GFP_ATOMIC);
+		tmp = kmalloc(entry->length, GFP_KERNEL);
 		if (!tmp) {
 			ret = -ENOMEM;
 			goto freeentry;