[PATCH RFC] powerpc/pseries: exploit H_PAGE_SET_UNUSED for partition migration

[Nathan Lynch via B4 Relay <devnull+nathanl.linux.ibm.com@kernel.org>]

From: Nathan Lynch <nathanl@linux.ibm.com>

Although the H_PAGE_INIT hcall's H_PAGE_SET_UNUSED historically has
been tied to the cooperative memory overcommit (CMO) platform feature,
the flag also is treated by the PowerVM hypervisor as a hint that the
page contents need not be copied to the destination during a live
partition migration.

Use the "ibm,migratable-partition" root node property to determine
whether this partition/guest can be migrated. Mark freed pages unused
if so (or if CMO is in use, as before).

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
---
Several things yet to improve here:

* powerpc's arch_free_page()/HAVE_ARCH_FREE_PAGE should be decoupled
  from CONFIG_PPC_SMLPAR.

* powerpc's arch_free_page() could be made to use a static key if
  justified.

* I have not yet measured the overhead this introduces, nor have I
  measured the benefit to a live migration.

To date, I have smoke tested it by doing a live migration and
performing a build on a kernel with the change, to ensure it doesn't
introduce obvious memory corruption or anything. It hasn't blown up
yet :-)

This will be a possibly significant behavior change in that we will be
flagging pages unused where we typically did not before. Until now,
having CMO enabled was the only way to do this, and I don't think that
feature is used all that much?

Posting this as RFC to see if there are any major concerns.
---
 arch/powerpc/platforms/pseries/lpar.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index 4561667832ed..b264371d8e12 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c
@@ -16,6 +16,7 @@
 #include <linux/export.h>
 #include <linux/jump_label.h>
 #include <linux/delay.h>
+#include <linux/of.h>
 #include <linux/stop_machine.h>
 #include <linux/spinlock.h>
 #include <linux/cpuhotplug.h>
@@ -1772,17 +1773,25 @@ static void pSeries_set_page_state(struct page *page, int order,
 	}
 }
 
+static bool migratable_partition;
+
 void arch_free_page(struct page *page, int order)
 {
-	if (radix_enabled())
-		return;
-	if (!cmo_free_hint_flag || !firmware_has_feature(FW_FEATURE_CMO))
-		return;
-
-	pSeries_set_page_state(page, order, H_PAGE_SET_UNUSED);
+	if (migratable_partition ||
+	    (firmware_has_feature(FW_FEATURE_CMO) && cmo_free_hint_flag))
+		pSeries_set_page_state(page, order, H_PAGE_SET_UNUSED);
 }
 EXPORT_SYMBOL(arch_free_page);
 
+static int __init check_migratable_partition(void)
+{
+	struct device_node *root = of_find_node_by_path("/");
+	migratable_partition = !!of_find_property(root, "ibm,migratable-partition", NULL);
+	of_node_put(root);
+	return 0;
+}
+machine_device_initcall(pseries, check_migratable_partition);
+
 #endif /* CONFIG_PPC_SMLPAR */
 #endif /* CONFIG_PPC_BOOK3S_64 */
 

---
base-commit: 44a1aad2fe6c10bfe0589d8047057b10a4c18a19
change-id: 20240111-h_page_set_unused-for-lpm-d9027c672fe8

Best regards,
-- 
Nathan Lynch <nathanl@linux.ibm.com>