FROMGIT: mm/vmalloc: Add override for lazy vunmap

Add an interface for arch code to disable lazy vunmap by forcing the
threshold to zero. This might be interesting for debugging/testing in
general, but primarily helps a horrible situation which needs to
guarantee that vmalloc aliases are up-to-date from atomic context,
wherein the only practical solution is to never let them get stale in
the first place.

Bug: 223346425
(cherry picked from commit 2a34c1503b85f49dd472dfd932dfcd16cab8ee8a
 https://git.gitlab.arm.com/linux-arm/linux-rm.git arm64/2454944)
Change-Id: I694523564357b4c43d30c129af1e89fd803824d3
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Beata Michalska <beata.michalska@arm.com>

include/linux/vmalloc.h

@@ -245,4 +245,8 @@ pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
 int register_vmap_purge_notifier(struct notifier_block *nb);
 int unregister_vmap_purge_notifier(struct notifier_block *nb);
 
+#ifndef arch_disable_lazy_vunmap
+#define arch_disable_lazy_vunmap false
+#endif
+
 #endif /* _LINUX_VMALLOC_H */

mm/vmalloc.c

@@ -1297,6 +1297,9 @@ static unsigned long lazy_max_pages(void)
 {
 	unsigned int log;
 
+	if (arch_disable_lazy_vunmap)
+		return 0;
+
 	log = fls(num_online_cpus());
 
 	return log * (32UL * 1024 * 1024 / PAGE_SIZE);