diff --git a/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h b/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
index c8cbb1e..72f1d91 100644
--- a/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
+++ b/fs_mgr/libsnapshot/include/libsnapshot/snapshot.h
@@ -148,7 +148,11 @@
     // /data is mounted.
     //
     // If a merge is in progress, this function will block until the merge is
-    // completed. If a merge or update was cancelled, this will clean up any
+    // completed.
+    //    - Callback is called periodically during the merge. If callback()
+    //      returns false during the merge, ProcessUpdateState() will pause
+    //      and returns Merging.
+    // If a merge or update was cancelled, this will clean up any
     // update artifacts and return.
     //
     // Note that after calling this, GetUpdateState() may still return that a
@@ -168,7 +172,7 @@
     //
     // The optional callback allows the caller to periodically check the
     // progress with GetUpdateState().
-    UpdateState ProcessUpdateState(const std::function<void()>& callback = {},
+    UpdateState ProcessUpdateState(const std::function<bool()>& callback = {},
                                    const std::function<bool()>& before_cancel = {});
 
     // Initiate the merge if necessary, then wait for the merge to finish.
diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp
index b3aa9f0..7a4f3dc 100644
--- a/fs_mgr/libsnapshot/snapshot.cpp
+++ b/fs_mgr/libsnapshot/snapshot.cpp
@@ -794,7 +794,7 @@
 // Note that when a merge fails, we will *always* try again to complete the
 // merge each time the device boots. There is no harm in doing so, and if
 // the problem was transient, we might manage to get a new outcome.
-UpdateState SnapshotManager::ProcessUpdateState(const std::function<void()>& callback,
+UpdateState SnapshotManager::ProcessUpdateState(const std::function<bool()>& callback,
                                                 const std::function<bool()>& before_cancel) {
     while (true) {
         UpdateState state = CheckMergeState(before_cancel);
@@ -807,8 +807,8 @@
             return state;
         }
 
-        if (callback) {
-            callback();
+        if (callback && !callback()) {
+            return state;
         }
 
         // This wait is not super time sensitive, so we have a relatively
@@ -2410,13 +2410,14 @@
     SnapshotMergeStats merge_stats(*this);
 
     unsigned int last_progress = 0;
-    auto callback = [&]() -> void {
+    auto callback = [&]() -> bool {
         double progress;
         GetUpdateState(&progress);
         if (last_progress < static_cast<unsigned int>(progress)) {
             last_progress = progress;
             LOG(INFO) << "Waiting for merge to complete: " << last_progress << "%.";
         }
+        return true;  // continue
     };
 
     LOG(INFO) << "Waiting for any previous merge request to complete. "
@@ -2512,7 +2513,10 @@
         return false;
     }
 
-    UpdateState state = ProcessUpdateState(callback);
+    UpdateState state = ProcessUpdateState([&]() -> bool {
+        callback();
+        return true;
+    });
     LOG(INFO) << "Update state in recovery: " << state;
     switch (state) {
         case UpdateState::MergeFailed: