ceph: introduce global empty snap context

commit 97c85a82 upstream.

Current snaphost code does not properly handle moving inode from one
empty snap realm to another empty snap realm. After changing inode's
snap realm, some dirty pages' snap context can be not equal to inode's
i_head_snap. This can trigger BUG() in ceph_put_wrbuffer_cap_refs()

The fix is introduce a global empty snap context for all empty snap
realm. This avoids triggering the BUG() for filesystem with no snapshot.

Fixes: http://tracker.ceph.com/issues/9928Signed-off-by: Yan, Zheng <zyan@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@redhat.com>
[bwh: Backported to 3.2:
 - Adjust context
 - As we don't have ceph_create_snap_context(), open-code it in
   ceph_snap_init()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/ceph/snap.c

@@ -288,6 +288,9 @@ static int cmpu64_rev(const void *a, const void *b)
 	return 0;
 }
 
+
+static struct ceph_snap_context *empty_snapc;
+
 /*
  * build the snap context for a given realm.
  */
@@ -329,6 +332,12 @@ static int build_snap_context(struct ceph_snap_realm *realm)
 		return 0;
 	}
 
+	if (num == 0 && realm->seq == empty_snapc->seq) {
+		ceph_get_snap_context(empty_snapc);
+		snapc = empty_snapc;
+		goto done;
+	}
+
 	/* alloc new snap context */
 	err = -ENOMEM;
 	if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
@@ -364,6 +373,7 @@ static int build_snap_context(struct ceph_snap_realm *realm)
 	dout("build_snap_context %llx %p: %p seq %lld (%d snaps)\n",
 	     realm->ino, realm, snapc, snapc->seq, snapc->num_snaps);
 
+done:
 	if (realm->cached_context)
 		ceph_put_snap_context(realm->cached_context);
 	realm->cached_context = snapc;
@@ -465,6 +475,9 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 		   cap_snap.  lucky us. */
 		dout("queue_cap_snap %p already pending\n", inode);
 		kfree(capsnap);
+	} else if (ci->i_snap_realm->cached_context == empty_snapc) {
+		dout("queue_cap_snap %p empty snapc\n", inode);
+		kfree(capsnap);
 	} else if (dirty & (CEPH_CAP_AUTH_EXCL|CEPH_CAP_XATTR_EXCL|
 			    CEPH_CAP_FILE_EXCL|CEPH_CAP_FILE_WR)) {
 		struct ceph_snap_context *snapc = ci->i_head_snapc;
@@ -927,5 +940,17 @@ void ceph_handle_snap(struct ceph_mds_client *mdsc,
 	return;
 }
 
+int __init ceph_snap_init(void)
+{
+	empty_snapc = kzalloc(sizeof(struct ceph_snap_context), GFP_NOFS);
+	if (!empty_snapc)
+		return -ENOMEM;
+	atomic_set(&empty_snapc->nref, 1);
+	empty_snapc->seq = 1;
+	return 0;
+}
 
-
+void ceph_snap_exit(void)
+{
+	ceph_put_snap_context(empty_snapc);
+}

fs/ceph/super.c

@@ -911,14 +911,20 @@ static int __init init_ceph(void)
 	if (ret)
 		goto out;
 
-	ret = register_filesystem(&ceph_fs_type);
+	ret = ceph_snap_init();
 	if (ret)
 		goto out_icache;
 
+	ret = register_filesystem(&ceph_fs_type);
+	if (ret)
+		goto out_snap;
+
 	pr_info("loaded (mds proto %d)\n", CEPH_MDSC_PROTOCOL);
 
 	return 0;
 
+out_snap:
+	ceph_snap_exit();
 out_icache:
 	destroy_caches();
 out:
@@ -929,6 +935,7 @@ static void __exit exit_ceph(void)
 {
 	dout("exit_ceph\n");
 	unregister_filesystem(&ceph_fs_type);
+	ceph_snap_exit();
 	destroy_caches();
 }
 

fs/ceph/super.h

@@ -677,6 +677,8 @@ extern void ceph_queue_cap_snap(struct ceph_inode_info *ci);
 extern int __ceph_finish_cap_snap(struct ceph_inode_info *ci,
 				  struct ceph_cap_snap *capsnap);
 extern void ceph_cleanup_empty_realms(struct ceph_mds_client *mdsc);
+extern int ceph_snap_init(void);
+extern void ceph_snap_exit(void);
 
 /*
  * a cap_snap is "pending" if it is still awaiting an in-progress