diff --git a/infinitepush/__init__.py b/infinitepush/__init__.py
--- a/infinitepush/__init__.py
+++ b/infinitepush/__init__.py
@@ -1078,7 +1078,12 @@
                                    True)
             elif part.type == scratchbookmarksparttype:
                 # Save this for later processing. Details below.
-                scratchbookpart = part
+                #
+                # Upstream https://phab.mercurial-scm.org/D1389 and its
+                # follow-ups stop part.seek support to reduce memory usage
+                # (https://bz.mercurial-scm.org/5691). So we need to copy
+                # the part so it can be consumed later.
+                scratchbookpart = bundleparts.copiedpart(part)
             else:
                 if handleallparts or part.type in partforwardingwhitelist:
                     # Ideally we would not process any parts, and instead just
@@ -1128,7 +1133,7 @@
     # processed after the main bundle has been stored, so that any commits it
     # references are available in the store.
     if scratchbookpart:
-        bundle2._processpart(op, part)
+        bundle2._processpart(op, scratchbookpart)
 
 def storebundle(op, params, bundlefile):
     log = _getorcreateinfinitepushlogger(op)
diff --git a/infinitepush/bundleparts.py b/infinitepush/bundleparts.py
--- a/infinitepush/bundleparts.py
+++ b/infinitepush/bundleparts.py
@@ -13,6 +13,7 @@
     error,
     extensions,
     revsetlang,
+    util,
 )
 from mercurial.i18n import _
 
@@ -112,3 +113,27 @@
     except KeyError:
         # Ignore if lfs extension is not enabled
         return
+
+class copiedpart(object):
+    """a copy of unbundlepart content that can be consumed later"""
+
+    def __init__(self, part):
+        # copy "public properties"
+        self.type = part.type
+        self.id = part.id
+        self.mandatory = part.mandatory
+        self.mandatoryparams = part.mandatoryparams
+        self.advisoryparams = part.advisoryparams
+        self.params = part.params
+        self.mandatorykeys = part.mandatorykeys
+        # copy the buffer
+        self._io = util.stringio(part.read())
+
+    def consume(self):
+        return
+
+    def read(self, size=None):
+        if size is None:
+            return self._io.read()
+        else:
+            return self._io.read(size)