ceph: avoid sending unnessesary FLUSHSNAP message

when a snap notification contains no new snapshot, we can avoid
sending FLUSHSNAP message to MDS. But we still need to create
cap_snap in some case because it's required by write path and
page writeback path
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/caps.c

@@ -1297,11 +1297,8 @@ void __ceph_flush_snaps(struct ceph_inode_info *ci,
 		if (capsnap->dirty_pages || capsnap->writing)
 			break;
 
-		/*
-		 * if cap writeback already occurred, we should have dropped
-		 * the capsnap in ceph_put_wrbuffer_cap_refs.
-		 */
-		BUG_ON(capsnap->dirty == 0);
+		/* should be removed by ceph_try_drop_cap_snap() */
+		BUG_ON(!capsnap->need_flush);
 
 		/* pick mds, take s_mutex */
 		if (ci->i_auth_cap == NULL) {
@@ -2347,6 +2344,27 @@ void ceph_get_cap_refs(struct ceph_inode_info *ci, int caps)
 	spin_unlock(&ci->i_ceph_lock);
 }
 
+
+/*
+ * drop cap_snap that is not associated with any snapshot.
+ * we don't need to send FLUSHSNAP message for it.
+ */
+static int ceph_try_drop_cap_snap(struct ceph_cap_snap *capsnap)
+{
+	if (!capsnap->need_flush &&
+	    !capsnap->writing && !capsnap->dirty_pages) {
+
+		dout("dropping cap_snap %p follows %llu\n",
+		     capsnap, capsnap->follows);
+		ceph_put_snap_context(capsnap->context);
+		list_del(&capsnap->ci_item);
+		list_del(&capsnap->flushing_item);
+		ceph_put_cap_snap(capsnap);
+		return 1;
+	}
+	return 0;
+}
+
 /*
  * Release cap refs.
  *
@@ -2360,7 +2378,6 @@ void ceph_put_cap_refs(struct ceph_inode_info *ci, int had)
 {
 	struct inode *inode = &ci->vfs_inode;
 	int last = 0, put = 0, flushsnaps = 0, wake = 0;
-	struct ceph_cap_snap *capsnap;
 
 	spin_lock(&ci->i_ceph_lock);
 	if (had & CEPH_CAP_PIN)
@@ -2382,17 +2399,17 @@ void ceph_put_cap_refs(struct ceph_inode_info *ci, int had)
 	if (had & CEPH_CAP_FILE_WR)
 		if (--ci->i_wr_ref == 0) {
 			last++;
-			if (!list_empty(&ci->i_cap_snaps)) {
-				capsnap = list_first_entry(&ci->i_cap_snaps,
-						     struct ceph_cap_snap,
-						     ci_item);
-				if (capsnap->writing) {
-					capsnap->writing = 0;
-					flushsnaps =
-						__ceph_finish_cap_snap(ci,
-								       capsnap);
-					wake = 1;
-				}
+			if (__ceph_have_pending_cap_snap(ci)) {
+				struct ceph_cap_snap *capsnap =
+					list_last_entry(&ci->i_cap_snaps,
+							struct ceph_cap_snap,
+							ci_item);
+				capsnap->writing = 0;
+				if (ceph_try_drop_cap_snap(capsnap))
+					put++;
+				else if (__ceph_finish_cap_snap(ci, capsnap))
+					flushsnaps = 1;
+				wake = 1;
 			}
 			if (ci->i_wrbuffer_ref_head == 0 &&
 			    ci->i_dirty_caps == 0 &&
@@ -2416,7 +2433,7 @@ void ceph_put_cap_refs(struct ceph_inode_info *ci, int had)
 		ceph_flush_snaps(ci);
 	if (wake)
 		wake_up_all(&ci->i_cap_wq);
-	if (put)
+	while (put-- > 0)
 		iput(inode);
 }
 
@@ -2467,25 +2484,15 @@ void ceph_put_wrbuffer_cap_refs(struct ceph_inode_info *ci, int nr,
 		capsnap->dirty_pages -= nr;
 		if (capsnap->dirty_pages == 0) {
 			complete_capsnap = 1;
-			if (capsnap->dirty == 0)
-				/* cap writeback completed before we created
-				 * the cap_snap; no FLUSHSNAP is needed */
-				drop_capsnap = 1;
+			drop_capsnap = ceph_try_drop_cap_snap(capsnap);
 		}
 		dout("put_wrbuffer_cap_refs on %p cap_snap %p "
-		     " snap %lld %d/%d -> %d/%d %s%s%s\n",
+		     " snap %lld %d/%d -> %d/%d %s%s\n",
 		     inode, capsnap, capsnap->context->seq,
 		     ci->i_wrbuffer_ref+nr, capsnap->dirty_pages + nr,
 		     ci->i_wrbuffer_ref, capsnap->dirty_pages,
 		     last ? " (wrbuffer last)" : "",
-		     complete_capsnap ? " (complete capsnap)" : "",
-		     drop_capsnap ? " (drop capsnap)" : "");
-		if (drop_capsnap) {
-			ceph_put_snap_context(capsnap->context);
-			list_del(&capsnap->ci_item);
-			list_del(&capsnap->flushing_item);
-			ceph_put_cap_snap(capsnap);
-		}
+		     complete_capsnap ? " (complete capsnap)" : "");
 	}
 
 	spin_unlock(&ci->i_ceph_lock);

fs/ceph/snap.c

@@ -436,6 +436,14 @@ static int dup_array(u64 **dst, __le64 *src, u32 num)
 	return 0;
 }
 
+static bool has_new_snaps(struct ceph_snap_context *o,
+			  struct ceph_snap_context *n)
+{
+	if (n->num_snaps == 0)
+		return false;
+	/* snaps are in descending order */
+	return n->snaps[0] > o->seq;
+}
 
 /*
  * When a snapshot is applied, the size/mtime inode metadata is queued
@@ -455,7 +463,7 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 {
 	struct inode *inode = &ci->vfs_inode;
 	struct ceph_cap_snap *capsnap;
-	struct ceph_snap_context *old_snapc;
+	struct ceph_snap_context *old_snapc, *new_snapc;
 	int used, dirty;
 
 	capsnap = kzalloc(sizeof(*capsnap), GFP_NOFS);
@@ -469,6 +477,7 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 	dirty = __ceph_caps_dirty(ci);
 
 	old_snapc = ci->i_head_snapc;
+	new_snapc = ci->i_snap_realm->cached_context;
 
 	/*
 	 * If there is a write in progress, treat that as a dirty Fw,
@@ -486,20 +495,37 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 		dout("queue_cap_snap %p already pending\n", inode);
 		goto update_snapc;
 	}
-	if (ci->i_snap_realm->cached_context == ceph_empty_snapc) {
-		dout("queue_cap_snap %p empty snapc\n", inode);
-		goto update_snapc;
-	}
-	if (!(dirty & (CEPH_CAP_AUTH_EXCL|CEPH_CAP_XATTR_EXCL|
-		       CEPH_CAP_FILE_EXCL|CEPH_CAP_FILE_WR))) {
+	if (ci->i_wrbuffer_ref_head == 0 &&
+	    !(dirty & (CEPH_CAP_ANY_EXCL|CEPH_CAP_FILE_WR))) {
 		dout("queue_cap_snap %p nothing dirty|writing\n", inode);
 		goto update_snapc;
 	}
 
 	BUG_ON(!old_snapc);
 
-	dout("queue_cap_snap %p cap_snap %p queuing under %p %s\n",
-	     inode, capsnap, old_snapc, ceph_cap_string(dirty));
+	/*
+	 * There is no need to send FLUSHSNAP message to MDS if there is
+	 * no new snapshot. But when there is dirty pages or on-going
+	 * writes, we still need to create cap_snap. cap_snap is needed
+	 * by the write path and page writeback path.
+	 *
+	 * also see ceph_try_drop_cap_snap()
+	 */
+	if (has_new_snaps(old_snapc, new_snapc)) {
+		if (dirty & (CEPH_CAP_ANY_EXCL|CEPH_CAP_FILE_WR))
+			capsnap->need_flush = true;
+	} else {
+		if (!(used & CEPH_CAP_FILE_WR) &&
+		    ci->i_wrbuffer_ref_head == 0) {
+			dout("queue_cap_snap %p "
+			     "no new_snap|dirty_page|writing\n", inode);
+			goto update_snapc;
+		}
+	}
+
+	dout("queue_cap_snap %p cap_snap %p queuing under %p %s %s\n",
+	     inode, capsnap, old_snapc, ceph_cap_string(dirty),
+	     capsnap->need_flush ? "" : "no_flush");
 	ihold(inode);
 
 	atomic_set(&capsnap->nref, 1);
@@ -549,9 +575,8 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 
 update_snapc:
 	if (ci->i_head_snapc) {
-		ci->i_head_snapc = ceph_get_snap_context(
-				ci->i_snap_realm->cached_context);
-		dout(" new snapc is %p\n", ci->i_head_snapc);
+		ci->i_head_snapc = ceph_get_snap_context(new_snapc);
+		dout(" new snapc is %p\n", new_snapc);
 	}
 	spin_unlock(&ci->i_ceph_lock);
 

fs/ceph/super.h

@@ -164,6 +164,7 @@ struct ceph_cap_snap {
 	int writing;   /* a sync write is still in progress */
 	int dirty_pages;     /* dirty pages awaiting writeback */
 	bool inline_data;
+	bool need_flush;
 };
 
 static inline void ceph_put_cap_snap(struct ceph_cap_snap *capsnap)
@@ -719,8 +720,8 @@ extern void ceph_snap_exit(void);
 static inline bool __ceph_have_pending_cap_snap(struct ceph_inode_info *ci)
 {
 	return !list_empty(&ci->i_cap_snaps) &&
-		list_entry(ci->i_cap_snaps.prev, struct ceph_cap_snap,
-			   ci_item)->writing;
+	       list_last_entry(&ci->i_cap_snaps, struct ceph_cap_snap,
+			       ci_item)->writing;
 }
 
 /* inode.c */