ANDROID: mm/vmalloc: Add override for lazy vunmap

Add an interface 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-dev)
Change-Id: I12fbbe3903f76a028ceea91ed078f0de2abe3815
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[BM: Convert to a flag that can be explicitly modified at runtime
     instead of relying on arch specific bits]
Signed-off-by: Beata Michalska <beata.michalska@arm.com>

include/linux/vmalloc.h

@@ -245,4 +245,7 @@ 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);
 
+/* Allow disabling lazy TLB flushing */
+extern bool lazy_vunmap_enable;
+
 #endif /* _LINUX_VMALLOC_H */

mm/vmalloc.c

@@ -1277,6 +1277,7 @@ int unregister_vmap_purge_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL_GPL(unregister_vmap_purge_notifier);
 
+bool lazy_vunmap_enable  __read_mostly = true;
 /*
  * lazy_max_pages is the maximum amount of virtual address space we gather up
  * before attempting to purge with a TLB flush.
@@ -1297,6 +1298,9 @@ static unsigned long lazy_max_pages(void)
 {
 	unsigned int log;
 
+	if (!lazy_vunmap_enable)
+		return 0;
+
 	log = fls(num_online_cpus());
 
 	return log * (32UL * 1024 * 1024 / PAGE_SIZE);