arm64/mm/hotplug: Enable MEM_OFFLINE event handling

This enables MEM_OFFLINE memory event handling. It will help intercept any
possible error condition such as if boot memory some how still got offlined
even after an explicit notifier failure, potentially by a future change in
generic hot plug framework. This would help detect such scenarios and help
debug further. While here, also call out the first section being attempted
for offline or got offlined.
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Steve Capper <steve.capper@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/1604896137-16644-3-git-send-email-anshuman.khandual@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/mmu.c

@@ -1493,13 +1493,43 @@ static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
 	unsigned long end_pfn = arg->start_pfn + arg->nr_pages;
 	unsigned long pfn = arg->start_pfn;
 
-	if (action != MEM_GOING_OFFLINE)
+	if ((action != MEM_GOING_OFFLINE) && (action != MEM_OFFLINE))
 		return NOTIFY_OK;
 
 	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+		unsigned long start = PFN_PHYS(pfn);
+		unsigned long end = start + (1UL << PA_SECTION_SHIFT);
+
 		ms = __pfn_to_section(pfn);
-		if (early_section(ms))
+		if (!early_section(ms))
+			continue;
+
+		if (action == MEM_GOING_OFFLINE) {
+			/*
+			 * Boot memory removal is not supported. Prevent
+			 * it via blocking any attempted offline request
+			 * for the boot memory and just report it.
+			 */
+			pr_warn("Boot memory [%lx %lx] offlining attempted\n", start, end);
 			return NOTIFY_BAD;
+		} else if (action == MEM_OFFLINE) {
+			/*
+			 * This should have never happened. Boot memory
+			 * offlining should have been prevented by this
+			 * very notifier. Probably some memory removal
+			 * procedure might have changed which would then
+			 * require further debug.
+			 */
+			pr_err("Boot memory [%lx %lx] offlined\n", start, end);
+
+			/*
+			 * Core memory hotplug does not process a return
+			 * code from the notifier for MEM_OFFLINE events.
+			 * The error condition has been reported. Return
+			 * from here as if ignored.
+			 */
+			return NOTIFY_DONE;
+		}
 	}
 	return NOTIFY_OK;
 }