dm: use noio when sending kobject event

kobject_uevent may allocate memory and it may be called while there are dm
devices suspended. The allocation may recurse into a suspended device,
causing a deadlock. We must set the noio flag when sending a uevent.

The observed deadlock was reported here:
https://www.redhat.com/archives/dm-devel/2020-March/msg00025.html

Reported-by: Khazhismel Kumykov <[email protected]>
Reported-by: Tahsin Erdogan <[email protected]>
Reported-by: Gabriel Krisman Bertazi <[email protected]>
Signed-off-by: Mikulas Patocka <[email protected]>
Cc: [email protected]
Signed-off-by: Mike Snitzer <[email protected]>

Author: Mikulas Patocka

drivers/md/dm.c

@@ -12,6 +12,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/sched/mm.h>
 #include <linux/sched/signal.h>
 #include <linux/blkpg.h>
 #include <linux/bio.h>
@@ -2939,17 +2940,25 @@ EXPORT_SYMBOL_GPL(dm_internal_resume_fast);
 int dm_kobject_uevent(struct mapped_device *md, enum kobject_action action,
 		       unsigned cookie)
 {
+	int r;
+	unsigned noio_flag;
 	char udev_cookie[DM_COOKIE_LENGTH];
 	char *envp[] = { udev_cookie, NULL };
 
+	noio_flag = memalloc_noio_save();
+
 	if (!cookie)
-		return kobject_uevent(&disk_to_dev(md->disk)->kobj, action);
+		r = kobject_uevent(&disk_to_dev(md->disk)->kobj, action);
 	else {
 		snprintf(udev_cookie, DM_COOKIE_LENGTH, "%s=%u",
 			 DM_COOKIE_ENV_VAR_NAME, cookie);
-		return kobject_uevent_env(&disk_to_dev(md->disk)->kobj,
-					  action, envp);
+		r = kobject_uevent_env(&disk_to_dev(md->disk)->kobj,
+				       action, envp);
 	}
+
+	memalloc_noio_restore(noio_flag);
+
+	return r;
 }
 
 uint32_t dm_next_uevent_seq(struct mapped_device *md)