[PATCH 3.16.y-ckt 52/58] ceph: introduce global empty snap context

[Luis Henriques]

3.16.7-ckt7 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Yan, Zheng" <zyan@xxxxxxxxxx>

commit 97c85a828f36bbfffe9d77b977b65a5872b6cad4 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/9928

Signed-off-by: Yan, Zheng <zyan@xxxxxxxxxx>
Reviewed-by: Ilya Dryomov <idryomov@xxxxxxxxxx>
Cc: Markus Blank-Burian <burian@xxxxxxxxxxx>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 fs/ceph/snap.c  | 26 +++++++++++++++++++++++++-
 fs/ceph/super.c | 10 ++++++++--
 fs/ceph/super.h |  2 ++
 3 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
index f01645a27752..fad6f7dfab19 100644
--- a/fs/ceph/snap.c
+++ b/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.
  */
@@ -328,6 +331,12 @@ 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))
@@ -365,6 +374,7 @@ static int build_snap_context(struct ceph_snap_realm *realm)
 	     realm->ino, realm, snapc, snapc->seq,
 	     (unsigned int) snapc->num_snaps);
 
+done:
 	if (realm->cached_context)
 		ceph_put_snap_context(realm->cached_context);
 	realm->cached_context = snapc;
@@ -466,6 +476,9 @@ 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;
@@ -928,5 +941,16 @@ out:
 	return;
 }
 
+int __init ceph_snap_init(void)
+{
+	empty_snapc = ceph_create_snap_context(0, GFP_NOFS);
+	if (!empty_snapc)
+		return -ENOMEM;
+	empty_snapc->seq = 1;
+	return 0;
+}
 
-
+void ceph_snap_exit(void)
+{
+	ceph_put_snap_context(empty_snapc);
+}
diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index 06150fd745ac..8503ec130c15 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -1028,15 +1028,20 @@ static int __init init_ceph(void)
 
 	ceph_flock_init();
 	ceph_xattr_init();
+	ret = ceph_snap_init();
+	if (ret)
+		goto out_xattr;
 	ret = register_filesystem(&ceph_fs_type);
 	if (ret)
-		goto out_icache;
+		goto out_snap;
 
 	pr_info("loaded (mds proto %d)\n", CEPH_MDSC_PROTOCOL);
 
 	return 0;
 
-out_icache:
+out_snap:
+	ceph_snap_exit();
+out_xattr:
 	ceph_xattr_exit();
 	destroy_caches();
 out:
@@ -1047,6 +1052,7 @@ static void __exit exit_ceph(void)
 {
 	dout("exit_ceph\n");
 	unregister_filesystem(&ceph_fs_type);
+	ceph_snap_exit();
 	ceph_xattr_exit();
 	destroy_caches();
 }
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index 12b20744e386..b5b324a94cca 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -672,6 +672,8 @@ 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
-- 
2.1.4

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