diff --git a/examples/image/expected.json b/examples/image/expected.json
index 2369bb04..06ade67a 100644
--- a/examples/image/expected.json
+++ b/examples/image/expected.json
@@ -1,7 +1,7 @@
 {
   "arm64": {
     "boot-demo.fedora": {
-      "sha256": "b9e969a15ed13e19ba214fb0a4de0f6885a5c1f74801a15bc9fcd66e17cf0608"
+      "sha256": "76733eaf5e248b9d6004557c9bfc20f200df0bc7c66dd7a742bde969b52bbbcb"
     },
     "boot-demo.fedora[boot][initrd]": {
       "sha256": "401d67515ee8cfcd6b546fc6ab8424c0a53999baadeb391f33b66e9e213a293a"
@@ -10,33 +10,33 @@
       "sha256": "a71b9242d2cc8523cf686e48fb6900f019897b163d514c1382545ce78283ce52"
     },
     "boot-demo.fedora[boot][uki]": {
-      "sha256": "e915b71b81d5ef03bf4078044c53e3c7ad802e6a2af3e00219c33b9f9ec9bacd"
+      "sha256": "92b5ae9022761d17bb9fbfc6fdf0b9dbaf551a191c3829718cc444c369e68cd7"
     },
     "boot-demo.fedora[disk-manifest]": {
-      "sha256": "eab47e4d127462512cac17fef2ceaa9bb2fe020c1c380b5e6e18cf314f42425f"
+      "sha256": "db5f03add27b22caf1f1abf511fce3fba670d5ae8710067e00b02e145741a4b0"
     },
     "boot-demo.fedora[manifests][esp]": {
-      "sha256": "bb8884fd42395ef55807474458561d1d65fdee626b58bdcc87c6ccdd20ca851a"
+      "sha256": "c5e812abadbafbfb96605da4874e1d6c2cc3e5b395e2ca9e3cfae35828a64000"
     },
     "boot-demo.fedora[manifests][usr]": {
       "sha256": "834e933fdc09451253d4a442a5abb8276c06a9226829df0f9c83a3cfb13a260e"
     },
     "boot-demo.fedora[partitions][usr-verity]": {
-      "sha256": "27c1e8c3f77d817770ee5c62772c6e0ce3becd6ce01807ca81700326309fffd0"
+      "sha256": "dac9fe1669ead033931ce9331218b700519cd70831b167f77b4279f18b018607"
     },
     "boot-demo.fedora[partitions][usr]": {
-      "sha256": "4d5186256e2793e048f53e9933189bec13027b60ffac5935a532661e3232a02b"
+      "sha256": "5c87fc88591163307507e141bee6f14938a0ece666267861b78494a88d9ca665"
     },
     "boot-demo.fedora[roothash]": {
-      "sha256": "9407f66d558acf921d6df8faa5a5ae46e8bbb68ac624a8a852a1e3a3b1dc0ff9"
+      "sha256": "a414d18d018c7ab07db0584d97b644b44aebae7b0de42b5ed404cf6d507d9001"
     },
     "demo-ext.fedora": {
-      "sha256": "c9cc5f0e6a7dba50fa1ad00d8d0ae587baa68b36a749a7cb38e5f9b45b2b84ff"
+      "sha256": "11474dff6783589d9b888174917f0f62fdeba7140ee69f4a50b07adf9070cd83"
     }
   },
   "x86_64": {
     "boot-demo.fedora": {
-      "sha256": "f0fb38aa158869023693d0a24eb7aebd0a763c523be97dde3f9ccd9424468505"
+      "sha256": "c2cec78b3489475f18be9db1cfba8f7ffe47614910b51f7ece9d0bf92bf98fdb"
     },
     "boot-demo.fedora[boot][initrd]": {
       "sha256": "d0081aa88c2729fb5363d30eb815e3cdc563eb9c61a5c19e93413ca80a3a0933"
@@ -45,28 +45,28 @@
       "sha256": "50896468d533671538dea76a150529455c2712075a927665339ed755065913c6"
     },
     "boot-demo.fedora[boot][uki]": {
-      "sha256": "11107e4625c10a53b8930cdae594c66377cee85823280ca83cdc1a04ff62b242"
+      "sha256": "445a85f5425a11d8af6094491c44854b746c6ee6fb0c029134348c148a2adc28"
     },
     "boot-demo.fedora[disk-manifest]": {
-      "sha256": "56510a81e72126aa1d8b2db0c68de051596e698caf807ab51ada46604b18ca9d"
+      "sha256": "ece373a06978f121266654bb894cff92fd038e2f5dc5b4bd2db04edfcb250e29"
     },
     "boot-demo.fedora[manifests][esp]": {
-      "sha256": "b1f607284750b98ccef4138f3480052f22ef1281a13694f1ff3e5a156126a4bd"
+      "sha256": "370ca1974ee8a13d3f0e0fa4bd18df38b936d82ae5c101dc6261dd7f983b07b9"
     },
     "boot-demo.fedora[manifests][usr]": {
       "sha256": "5ba6f1c6690e971d62b97e17d2131cc28186c4992d1cfa27a1ccca6343590654"
     },
     "boot-demo.fedora[partitions][usr-verity]": {
-      "sha256": "1ac7cc46e34b01b516de5c44bd3ecd3a2584050b5ec8322792bb113d4908f42e"
+      "sha256": "704762d78ff960f9af9d4456b0a3aa7c97f5df163fddc758c65875ae212aaef5"
     },
     "boot-demo.fedora[partitions][usr]": {
-      "sha256": "62080ba93b9bcfd248ce15aa00427c408c64a804fa9d05fa10717b82adc1019b"
+      "sha256": "bee5f2667fd1e5b2e84bc1af73a7597a49d6a6f7e15783ed8233e286fefc7881"
     },
     "boot-demo.fedora[roothash]": {
-      "sha256": "b6e6fb44025de875fb06fa3a58d977ae1994db16c659ee3c356dd5e54f478e74"
+      "sha256": "6293f3204a7c57cd7f23b737aad9dac1b97eb5ea1c762038d065b4807bb430ef"
     },
     "demo-ext.fedora": {
-      "sha256": "453273998618ab5d0bb2e47f7b2b6c3b2e4f89e3af36887ac7a376585bdba6b5"
+      "sha256": "6c53c52b1dab63cb7e876f1ec45d8dcd0c16d0f9edb003471653db2df65c296a"
     }
   }
 }
diff --git a/image_format/disk.py b/image_format/disk.py
index 47120679..1ea512bb 100755
--- a/image_format/disk.py
+++ b/image_format/disk.py
@@ -368,11 +368,7 @@ def main(argv: list[str] | None = None) -> None:
         if splits:
             cmd.append("--split=yes")
         cmd += repart.key_arguments(spec["signing"])
-        # repart reads one variable per filesystem it formats and splits each on whitespace.
-        env = os.environ | {
-            f"SYSTEMD_REPART_MKFS_OPTIONS_{filesystem.upper()}": " ".join(options)
-            for filesystem, options in spec["mkfs_options"].items()
-        }
+        env = os.environ | repart.mkfs_environment(spec["mkfs_options"])
         result = subprocess.run([*cmd, str(disk)], check=True, env=env, stdout=subprocess.PIPE, text=True)
         rows: list[dict[str, Any]] = json.loads(result.stdout)
 
diff --git a/image_format/repart.py b/image_format/repart.py
index 62a0c5e1..ed904fc3 100644
--- a/image_format/repart.py
+++ b/image_format/repart.py
@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: Amutable GmbH <https://amutable.com/>
 # SPDX-License-Identifier: MPL-2.0
 
-"""What the drivers invoking systemd-repart share: signing key material, and reading its report back."""
+"""Signing keys, mkfs options and report parsing for the drivers that run systemd-repart."""
 
 from pathlib import Path
 from typing import Any, TypedDict
@@ -31,6 +31,20 @@ def key_arguments(signing: KeySpec | None) -> list[str]:
     return arguments
 
 
+def mkfs_environment(options: dict[str, list[str]]) -> dict[str, str]:
+    """Return the systemd-repart environment that passes `options` to mkfs, per filesystem."""
+    erofs = options.get("erofs", [])
+    # The kernel shares the page cache of identical files between erofs images mounted with
+    # inode_share, but only for images that record a digest of each file.
+    if not any(option.startswith("--xattr-inode-digest") for option in erofs):
+        erofs = [*erofs, "--xattr-inode-digest"]
+    # repart reads one variable per filesystem it formats and splits each on whitespace.
+    return {
+        f"SYSTEMD_REPART_MKFS_OPTIONS_{filesystem.upper()}": " ".join(arguments)
+        for filesystem, arguments in (options | {"erofs": erofs}).items()
+    }
+
+
 def write_root_hash(rows: list[dict[str, Any]], output: Path) -> None:
     """Write the verity root hash repart generated, as hex plus a newline.
 
diff --git a/image_format/repart_test.py b/image_format/repart_test.py
index 1b98b12b..03dd10b8 100644
--- a/image_format/repart_test.py
+++ b/image_format/repart_test.py
@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: Amutable GmbH <https://amutable.com/>
 # SPDX-License-Identifier: MPL-2.0
 
-"""Tests for reading repart's partition report back.
+"""Tests for the mkfs environment and for reading repart's partition report back.
 
     buck test tine//image_format:test
 
@@ -17,6 +17,29 @@
 HASH = "d3b20557d33fdb5d496b6faec48fbdf517db6f428dae1c362b59c16dac405bfe"
 
 
+class TestMkfsEnvironment(unittest.TestCase):
+    def test_records_erofs_digests_without_options(self) -> None:
+        self.assertEqual(
+            repart.mkfs_environment({}), {"SYSTEMD_REPART_MKFS_OPTIONS_EROFS": "--xattr-inode-digest"}
+        )
+
+    def test_appends_the_digest_to_erofs_options(self) -> None:
+        environment = repart.mkfs_environment({"erofs": ["-zzstd", "-C524288"], "vfat": ["--invariant"]})
+        self.assertEqual(
+            environment,
+            {
+                "SYSTEMD_REPART_MKFS_OPTIONS_EROFS": "-zzstd -C524288 --xattr-inode-digest",
+                "SYSTEMD_REPART_MKFS_OPTIONS_VFAT": "--invariant",
+            },
+        )
+
+    def test_keeps_a_digest_name_the_caller_picked(self) -> None:
+        environment = repart.mkfs_environment({"erofs": ["--xattr-inode-digest=user.digest"]})
+        self.assertEqual(
+            environment, {"SYSTEMD_REPART_MKFS_OPTIONS_EROFS": "--xattr-inode-digest=user.digest"}
+        )
+
+
 class TestWriteRootHash(unittest.TestCase):
     def _write(self, rows: list[dict[str, object]]) -> str:
         with tempfile.TemporaryDirectory(prefix="repart_test.") as scratch:
diff --git a/image_format/sysext.py b/image_format/sysext.py
index 63a0cb1a..f89775d7 100755
--- a/image_format/sysext.py
+++ b/image_format/sysext.py
@@ -51,7 +51,7 @@ class Spec(finalize.ImageSpec):
 
 # erofs dedup keys off the filesystem UUID, so combined with a stable seed these keep the DDI
 # reproducible.
-_MKFS_OPTIONS_EROFS = "--quiet -zzstd,level=3 -C524288 -Efragments,ztailpacking,dedupe"
+_MKFS_OPTIONS_EROFS = ["--quiet", "-zzstd,level=3", "-C524288", "-Efragments,ztailpacking,dedupe"]
 
 
 def _derived_seed(identity: str, name: str, release: dict[str, str]) -> uuid.UUID:
@@ -151,7 +151,7 @@ def main(argv: list[str] | None = None) -> None:
         else:
             cmd.append("--exclude-partitions=root-verity-sig")
         cmd.append(str(out))
-        env = os.environ | {"SYSTEMD_REPART_MKFS_OPTIONS_EROFS": _MKFS_OPTIONS_EROFS}
+        env = os.environ | repart.mkfs_environment({"erofs": _MKFS_OPTIONS_EROFS})
         result = subprocess.run(cmd, check=True, env=env, stdout=subprocess.PIPE, text=True)
         # The root hash is what a host reports for the merged extension
         # (/usr/.systemd-sysext/origin), so it is the DDI's identity to