arm64/mm/hotplug: Ensure early memory sections are all online

This adds a validation function that scans the entire boot memory and makes
sure that all early memory sections are online. This check is essential for
the memory notifier to work properly, as it cannot prevent any boot memory
from offlining, if all sections are not online to begin with. Although the
boot section scanning is selectively enabled with DEBUG_VM.
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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/1604896137-16644-4-git-send-email-anshuman.khandual@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/mmu.c

@@ -1538,6 +1538,53 @@ static struct notifier_block prevent_bootmem_remove_nb = {
 	.notifier_call = prevent_bootmem_remove_notifier,
 };
 
+/*
+ * This ensures that boot memory sections on the platform are online
+ * from early boot. Memory sections could not be prevented from being
+ * offlined, unless for some reason they are not online to begin with.
+ * This helps validate the basic assumption on which the above memory
+ * event notifier works to prevent boot memory section offlining and
+ * its possible removal.
+ */
+static void validate_bootmem_online(void)
+{
+	phys_addr_t start, end, addr;
+	struct mem_section *ms;
+	u64 i;
+
+	/*
+	 * Scanning across all memblock might be expensive
+	 * on some big memory systems. Hence enable this
+	 * validation only with DEBUG_VM.
+	 */
+	if (!IS_ENABLED(CONFIG_DEBUG_VM))
+		return;
+
+	for_each_mem_range(i, &start, &end) {
+		for (addr = start; addr < end; addr += (1UL << PA_SECTION_SHIFT)) {
+			ms = __pfn_to_section(PHYS_PFN(addr));
+
+			/*
+			 * All memory ranges in the system at this point
+			 * should have been marked as early sections.
+			 */
+			WARN_ON(!early_section(ms));
+
+			/*
+			 * Memory notifier mechanism here to prevent boot
+			 * memory offlining depends on the fact that each
+			 * early section memory on the system is initially
+			 * online. Otherwise a given memory section which
+			 * is already offline will be overlooked and can
+			 * be removed completely. Call out such sections.
+			 */
+			if (!online_section(ms))
+				pr_err("Boot memory [%llx %llx] is offline, can be removed\n",
+					addr, addr + (1UL << PA_SECTION_SHIFT));
+		}
+	}
+}
+
 static int __init prevent_bootmem_remove_init(void)
 {
 	int ret = 0;
@@ -1545,6 +1592,7 @@ static int __init prevent_bootmem_remove_init(void)
 	if (!IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
 		return ret;
 
+	validate_bootmem_online();
 	ret = register_memory_notifier(&prevent_bootmem_remove_nb);
 	if (ret)
 		pr_err("%s: Notifier registration failed %d\n", __func__, ret);