[PATCH v3] liveupdate: separate memfd support into LIVEUPDATE_MEMFD

[Pasha Tatashin <pasha.tatashin@soleen.com>]

Decouple memfd preservation support from the core Live Update
Orchestrator configuration.

Previously, enabling CONFIG_LIVEUPDATE forced a dependency on
CONFIG_SHMEM and unconditionally compiled memfd_luo.o. However, Live
Update may be used for purposes that do not require memfd-backed memory
preservation.

Introduce CONFIG_LIVEUPDATE_MEMFD to gate memfd_luo.o. This moves the
SHMEM and MEMFD_CREATE dependencies to the specific feature that needs
them, allowing the base LIVEUPDATE option to be selected independently
of shared memory support.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
---
Changes v3
- Addressed Mike's suggestion: added default LIVEUPDATE, and removed
  config from selftests.

 kernel/liveupdate/Kconfig | 17 ++++++++++++++++-
 mm/Makefile               |  2 +-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/liveupdate/Kconfig b/kernel/liveupdate/Kconfig
index d2aeaf13c3ac..1a8513f16ef7 100644
--- a/kernel/liveupdate/Kconfig
+++ b/kernel/liveupdate/Kconfig
@@ -54,7 +54,6 @@ config KEXEC_HANDOVER_ENABLE_DEFAULT
 config LIVEUPDATE
 	bool "Live Update Orchestrator"
 	depends on KEXEC_HANDOVER
-	depends on SHMEM
 	help
 	  Enable the Live Update Orchestrator. Live Update is a mechanism,
 	  typically based on kexec, that allows the kernel to be updated
@@ -73,4 +72,20 @@ config LIVEUPDATE
 
 	  If unsure, say N.
 
+config LIVEUPDATE_MEMFD
+	bool "Live update support for memfd"
+	depends on LIVEUPDATE
+	depends on MEMFD_CREATE
+	depends on SHMEM
+	default LIVEUPDATE
+	help
+	  Enable live update support for memfd regions. This allows preserving
+	  memfd-backed memory across kernel live updates.
+
+	  This can be used to back VM memory with memfds, allowing the guest
+	  memory to persist, or for other user workloads needing to preserve
+	  pages.
+
+	  If unsure, say N.
+
 endmenu
diff --git a/mm/Makefile b/mm/Makefile
index 9175f8cc6565..798f04233d71 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -100,7 +100,7 @@ obj-$(CONFIG_NUMA) += memory-tiers.o
 obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o
 obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
 obj-$(CONFIG_PAGE_COUNTER) += page_counter.o
-obj-$(CONFIG_LIVEUPDATE) += memfd_luo.o
+obj-$(CONFIG_LIVEUPDATE_MEMFD) += memfd_luo.o
 obj-$(CONFIG_MEMCG_V1) += memcontrol-v1.o
 obj-$(CONFIG_MEMCG) += memcontrol.o vmpressure.o
 ifdef CONFIG_SWAP
-- 
2.52.0.351.gbe84eed79e-goog