diff --git a/hgext/remotefilelog/README.md b/hgext/remotefilelog/README.md
--- a/hgext/remotefilelog/README.md
+++ b/hgext/remotefilelog/README.md
@@ -88,7 +88,9 @@
 
 4. Tags are not supported in completely shallow repos. If you use tags in your repo you will have to specify `excludepattern=.hgtags` in your client configuration to ensure that file is downloaded. The include/excludepattern settings are experimental at the moment and have yet to be deployed in a production environment.
 
-5. A few commands will be slower. `hg log <filename>` will be much slower since it has to walk the entire commit history instead of just the filelog. Use `hg log -f <filename>` instead, which remains very fast.
+5. Similarly, subrepositories should not be used with completely shallow repos. Use `excludepattern=.hgsub*` in your client configuration to ensure that the files are downloaded.
+
+6. A few commands will be slower. `hg log <filename>` will be much slower since it has to walk the entire commit history instead of just the filelog. Use `hg log -f <filename>` instead, which remains very fast.
 
 Contributing
 ============
diff --git a/hgext/remotefilelog/remotefilelog.py b/hgext/remotefilelog/remotefilelog.py
--- a/hgext/remotefilelog/remotefilelog.py
+++ b/hgext/remotefilelog/remotefilelog.py
@@ -245,11 +245,11 @@
     __bool__ = __nonzero__
 
     def __len__(self):
-        if self.filename == b'.hgtags':
-            # The length of .hgtags is used to fast path tag checking.
-            # remotefilelog doesn't support .hgtags since the entire .hgtags
-            # history is needed.  Use the excludepattern setting to make
-            # .hgtags a normal filelog.
+        if self.filename in (b'.hgtags', b'.hgsub', b'.hgsubstate'):
+            # Global tag and subrepository support require access to the
+            # file history for various performance sensitive operations.
+            # excludepattern should be used for repositories depending on
+            # those features to fallback to regular filelog.
             return 0
 
         raise RuntimeError(b"len not supported")
diff --git a/mercurial/exchange.py b/mercurial/exchange.py
--- a/mercurial/exchange.py
+++ b/mercurial/exchange.py
@@ -522,8 +522,16 @@
 
 def _checksubrepostate(pushop):
     """Ensure all outgoing referenced subrepo revisions are present locally"""
+
+    repo = pushop.repo
+
+    # If the repository does not use subrepos, skip the expensive
+    # manifest checks.
+    if not len(repo.file(b'.hgsub')) or not len(repo.file(b'.hgsubstate')):
+        return
+
     for n in pushop.outgoing.missing:
-        ctx = pushop.repo[n]
+        ctx = repo[n]
 
         if b'.hgsub' in ctx.manifest() and b'.hgsubstate' in ctx.files():
             for subpath in sorted(ctx.substate):
diff --git a/relnotes/next b/relnotes/next
--- a/relnotes/next
+++ b/relnotes/next
@@ -15,6 +15,8 @@
 
 == Backwards Compatibility Changes ==
 
+The remotefilelog extension now requires an appropiate excludepattern
+for subrepositories.
 
 == Internal API Changes ==