diff --git a/object-file.c b/object-file.c
index 3d27dc8dea..621b121bcb 100644
--- a/object-file.c
+++ b/object-file.c
@@ -32,6 +32,7 @@
 #include "packfile.h"
 #include "object-store.h"
 #include "promisor-remote.h"
+#include "submodule.h"
 
 /* The maximum size for an object header. */
 #define MAX_HEADER_LEN 32
@@ -1592,6 +1593,10 @@ static int do_oid_object_info_extended(struct repository *r,
 				break;
 		}
 
+		if (register_all_submodule_odb_as_alternates())
+			/* We added some alternates; retry */
+			continue;
+
 		/* Check if it is a missing object */
 		if (fetch_if_missing && repo_has_promisor_remote(r) &&
 		    !already_retried &&
diff --git a/submodule.c b/submodule.c
index 8e611fe1db..8fde90e906 100644
--- a/submodule.c
+++ b/submodule.c
@@ -165,6 +165,8 @@ void stage_updated_gitmodules(struct index_state *istate)
 		die(_("staging updated .gitmodules failed"));
 }
 
+static struct string_list added_submodule_odb_paths = STRING_LIST_INIT_NODUP;
+
 /* TODO: remove this function, use repo_submodule_init instead. */
 int add_submodule_odb(const char *path)
 {
@@ -178,12 +180,28 @@ int add_submodule_odb(const char *path)
 		ret = -1;
 		goto done;
 	}
-	add_to_alternates_memory(objects_directory.buf);
+	string_list_insert(&added_submodule_odb_paths,
+			   strbuf_detach(&objects_directory, NULL));
 done:
 	strbuf_release(&objects_directory);
 	return ret;
 }
 
+int register_all_submodule_odb_as_alternates(void)
+{
+	int i;
+	int ret = added_submodule_odb_paths.nr;
+
+	for (i = 0; i < added_submodule_odb_paths.nr; i++)
+		add_to_alternates_memory(added_submodule_odb_paths.items[i].string);
+	if (ret) {
+		string_list_clear(&added_submodule_odb_paths, 0);
+		if (git_env_bool("GIT_TEST_FATAL_REGISTER_SUBMODULE_ODB", 0))
+			BUG("register_all_submodule_odb_as_alternates() called");
+	}
+	return ret;
+}
+
 void set_diffopt_flags_from_submodule_config(struct diff_options *diffopt,
 					     const char *path)
 {
diff --git a/submodule.h b/submodule.h
index 84640c49c1..c252784bc2 100644
--- a/submodule.h
+++ b/submodule.h
@@ -97,7 +97,14 @@ int submodule_uses_gitfile(const char *path);
 #define SUBMODULE_REMOVAL_IGNORE_IGNORED_UNTRACKED (1<<2)
 int bad_to_remove_submodule(const char *path, unsigned flags);
 
+/*
+ * Call add_submodule_odb() to add the submodule at the given path to a list.
+ * When register_all_submodule_odb_as_alternates() is called, the object stores
+ * of all submodules in that list will be added as alternates in
+ * the_repository.
+ */
 int add_submodule_odb(const char *path);
+int register_all_submodule_odb_as_alternates(void);
 
 /*
  * Checks if there are submodule changes in a..b. If a is the null OID,
diff --git a/t/README b/t/README
index 9e70122302..8b67b4f00b 100644
--- a/t/README
+++ b/t/README
@@ -448,6 +448,16 @@ GIT_TEST_CHECKOUT_WORKERS=<n> overrides the 'checkout.workers' setting
 to <n> and 'checkout.thresholdForParallelism' to 0, forcing the
 execution of the parallel-checkout code.
 
+GIT_TEST_FATAL_REGISTER_SUBMODULE_ODB=<boolean>, when true, makes
+registering submodule ODBs as alternates a fatal action. Support for
+this environment variable can be removed once the migration to
+explicitly providing repositories when accessing submodule objects is
+complete (in which case we might want to replace this with a trace2
+call so that users can make it visible if accessing submodule objects
+without an explicit repository still happens) or needs to be abandoned
+for whatever reason (in which case the migrated codepaths still retain
+their performance benefits).
+
 Naming Tests
 ------------