ceph: maintain i_head_snapc when any caps are dirty, not just for data

We used to use i_head_snapc to keep track of which snapc the current epoch
of dirty data was dirtied under.  It is used by queue_cap_snap to set up
the cap_snap.  However, since we queue cap snaps for any dirty caps, not
just for dirty file data, we need to keep a valid i_head_snapc anytime
we have dirty|flushing caps.  This fixes a NULL pointer deref in
queue_cap_snap when writing back dirty caps without data (e.g.,
snaptest-authwb.sh).
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/addr.c

@@ -87,7 +87,7 @@ static int ceph_set_page_dirty(struct page *page)
 
 	/* dirty the head */
 	spin_lock(&inode->i_lock);
-	if (ci->i_wrbuffer_ref_head == 0)
+	if (ci->i_head_snapc == NULL)
 		ci->i_head_snapc = ceph_get_snap_context(snapc);
 	++ci->i_wrbuffer_ref_head;
 	if (ci->i_wrbuffer_ref == 0)
@@ -346,7 +346,7 @@ static struct ceph_snap_context *get_oldest_context(struct inode *inode,
 			break;
 		}
 	}
-	if (!snapc && ci->i_head_snapc) {
+	if (!snapc && ci->i_wrbuffer_ref_head) {
 		snapc = ceph_get_snap_context(ci->i_head_snapc);
 		dout(" head snapc %p has %d dirty pages\n",
 		     snapc, ci->i_wrbuffer_ref_head);

fs/ceph/caps.c

@@ -1143,6 +1143,10 @@ static int __send_cap(struct ceph_mds_client *mdsc, struct ceph_cap *cap,
 		for (i = 0; i < CEPH_CAP_BITS; i++)
 			if (flushing & (1 << i))
 				ci->i_cap_flush_tid[i] = flush_tid;
+
+		follows = ci->i_head_snapc->seq;
+	} else {
+		follows = 0;
 	}
 
 	keep = cap->implemented;
@@ -1156,7 +1160,6 @@ static int __send_cap(struct ceph_mds_client *mdsc, struct ceph_cap *cap,
 	mtime = inode->i_mtime;
 	atime = inode->i_atime;
 	time_warp_seq = ci->i_time_warp_seq;
-	follows = ci->i_snap_realm->cached_context->seq;
 	uid = inode->i_uid;
 	gid = inode->i_gid;
 	mode = inode->i_mode;
@@ -1332,7 +1335,11 @@ void __ceph_mark_dirty_caps(struct ceph_inode_info *ci, int mask)
 	     ceph_cap_string(was | mask));
 	ci->i_dirty_caps |= mask;
 	if (was == 0) {
-		dout(" inode %p now dirty\n", &ci->vfs_inode);
+		if (!ci->i_head_snapc)
+			ci->i_head_snapc = ceph_get_snap_context(
+				ci->i_snap_realm->cached_context);
+		dout(" inode %p now dirty snapc %p\n", &ci->vfs_inode,
+			ci->i_head_snapc);
 		BUG_ON(!list_empty(&ci->i_dirty_item));
 		spin_lock(&mdsc->cap_dirty_lock);
 		list_add(&ci->i_dirty_item, &mdsc->cap_dirty);
@@ -2190,7 +2197,9 @@ void ceph_put_wrbuffer_cap_refs(struct ceph_inode_info *ci, int nr,
 
 	if (ci->i_head_snapc == snapc) {
 		ci->i_wrbuffer_ref_head -= nr;
-		if (!ci->i_wrbuffer_ref_head) {
+		if (ci->i_wrbuffer_ref_head == 0 &&
+		    ci->i_dirty_caps == 0 && ci->i_flushing_caps == 0) {
+			BUG_ON(!ci->i_head_snapc);
 			ceph_put_snap_context(ci->i_head_snapc);
 			ci->i_head_snapc = NULL;
 		}
@@ -2483,6 +2492,11 @@ static void handle_cap_flush_ack(struct inode *inode, u64 flush_tid,
 			dout(" inode %p now clean\n", inode);
 			BUG_ON(!list_empty(&ci->i_dirty_item));
 			drop = 1;
+			if (ci->i_wrbuffer_ref_head == 0) {
+				BUG_ON(!ci->i_head_snapc);
+				ceph_put_snap_context(ci->i_head_snapc);
+				ci->i_head_snapc = NULL;
+			}
 		} else {
 			BUG_ON(list_empty(&ci->i_dirty_item));
 		}

fs/ceph/snap.c

@@ -458,6 +458,8 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 			     CEPH_CAP_FILE_EXCL|CEPH_CAP_FILE_WR))) {
 		struct ceph_snap_context *snapc = ci->i_head_snapc;
 
+		dout("queue_cap_snap %p cap_snap %p queuing under %p\n", inode,
+		     capsnap, snapc);
 		igrab(inode);
 		
 		atomic_set(&capsnap->nref, 1);
@@ -489,7 +491,9 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 		capsnap->dirty_pages = ci->i_wrbuffer_ref_head;
 		ci->i_wrbuffer_ref_head = 0;
 		capsnap->context = snapc;
-		ci->i_head_snapc = NULL;
+		ci->i_head_snapc =
+			ceph_get_snap_context(ci->i_snap_realm->cached_context);
+		dout(" new snapc is %p\n", ci->i_head_snapc);
 		list_add_tail(&capsnap->ci_item, &ci->i_cap_snaps);
 
 		if (used & CEPH_CAP_FILE_WR) {

fs/ceph/super.h

@@ -344,7 +344,8 @@ struct ceph_inode_info {
 	unsigned i_cap_exporting_issued;
 	struct ceph_cap_reservation i_cap_migration_resv;
 	struct list_head i_cap_snaps;   /* snapped state pending flush to mds */
-	struct ceph_snap_context *i_head_snapc;  /* set if wr_buffer_head > 0 */
+	struct ceph_snap_context *i_head_snapc;  /* set if wr_buffer_head > 0 or
+						    dirty|flushing caps */
 	unsigned i_snap_caps;           /* cap bits for snapped files */
 
 	int i_nr_by_mode[CEPH_FILE_MODE_NUM];  /* open file counts */