ceph: queue_cap_snap should always queue dirty context

This simplifies the calling convention, and fixes a bug where we queue a
capsnap with a context other than i_head_snapc (the one that matches the
dirty pages).  The result was a BUG at fs/ceph/caps.c:2178 on writeback
completion when a capsnap matching the writeback snapc could not be found.
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/snap.c

@@ -430,8 +430,7 @@ static int dup_array(u64 **dst, __le64 *src, int num)
  * Caller must hold snap_rwsem for read (i.e., the realm topology won't
  * change).
  */
-void ceph_queue_cap_snap(struct ceph_inode_info *ci,
-			 struct ceph_snap_context *snapc)
+void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 {
 	struct inode *inode = &ci->vfs_inode;
 	struct ceph_cap_snap *capsnap;
@@ -450,10 +449,11 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci,
 		   as no new writes are allowed to start when pending, so any
 		   writes in progress now were started before the previous
 		   cap_snap.  lucky us. */
-		dout("queue_cap_snap %p snapc %p seq %llu used %d"
-		     " already pending\n", inode, snapc, snapc->seq, used);
+		dout("queue_cap_snap %p already pending\n", inode);
 		kfree(capsnap);
 	} else if (ci->i_wrbuffer_ref_head || (used & CEPH_CAP_FILE_WR)) {
+		struct ceph_snap_context *snapc = ci->i_head_snapc;
+
 		igrab(inode);
 
 		atomic_set(&capsnap->nref, 1);
@@ -462,7 +462,6 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci,
 		INIT_LIST_HEAD(&capsnap->flushing_item);
 
 		capsnap->follows = snapc->seq - 1;
-		capsnap->context = ceph_get_snap_context(snapc);
 		capsnap->issued = __ceph_caps_issued(ci, NULL);
 		capsnap->dirty = __ceph_caps_dirty(ci);
 
@@ -479,7 +478,7 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci,
 		   snapshot. */
 		capsnap->dirty_pages = ci->i_wrbuffer_ref_head;
 		ci->i_wrbuffer_ref_head = 0;
-		ceph_put_snap_context(ci->i_head_snapc);
+		capsnap->context = snapc;
 		ci->i_head_snapc = NULL;
 		list_add_tail(&capsnap->ci_item, &ci->i_cap_snaps);
 
@@ -603,7 +602,7 @@ int ceph_update_snap_trace(struct ceph_mds_client *mdsc,
 				if (lastinode)
 					iput(lastinode);
 				lastinode = inode;
-				ceph_queue_cap_snap(ci, realm->cached_context);
+				ceph_queue_cap_snap(ci);
 				spin_lock(&realm->inodes_with_caps_lock);
 			}
 			spin_unlock(&realm->inodes_with_caps_lock);
@@ -825,8 +824,7 @@ void ceph_handle_snap(struct ceph_mds_client *mdsc,
 			spin_unlock(&realm->inodes_with_caps_lock);
 			spin_unlock(&inode->i_lock);
 
-			ceph_queue_cap_snap(ci,
-					    ci->i_snap_realm->cached_context);
+			ceph_queue_cap_snap(ci);
 
 			iput(inode);
 			continue;

fs/ceph/super.h

@@ -714,8 +714,7 @@ extern int ceph_update_snap_trace(struct ceph_mds_client *m,
 extern void ceph_handle_snap(struct ceph_mds_client *mdsc,
 			     struct ceph_mds_session *session,
 			     struct ceph_msg *msg);
-extern void ceph_queue_cap_snap(struct ceph_inode_info *ci,
-				struct ceph_snap_context *snapc);
+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);