powerpc/powernv: Allow memory that has been hot-removed to be hot-added

This patch allows the memory removed by memtrace to be readded to the
kernel. So now you don't have to reboot your system to add the memory
back to the kernel or to have a different amount of memory removed.
Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Tested-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/memtrace.c

@@ -177,8 +177,11 @@ static int memtrace_init_debugfs(void)
 
 		snprintf(ent->name, 16, "%08x", ent->nid);
 		dir = debugfs_create_dir(ent->name, memtrace_debugfs_dir);
-		if (!dir)
+		if (!dir) {
+			pr_err("Failed to create debugfs directory for node %d\n",
+				ent->nid);
 			return -1;
+		}
 
 		ent->dir = dir;
 		debugfs_create_file("trace", 0400, dir, ent, &memtrace_fops);
@@ -189,18 +192,93 @@ static int memtrace_init_debugfs(void)
 	return ret;
 }
 
+static int online_mem_block(struct memory_block *mem, void *arg)
+{
+	return device_online(&mem->dev);
+}
+
+/*
+ * Iterate through the chunks of memory we have removed from the kernel
+ * and attempt to add them back to the kernel.
+ */
+static int memtrace_online(void)
+{
+	int i, ret = 0;
+	struct memtrace_entry *ent;
+
+	for (i = memtrace_array_nr - 1; i >= 0; i--) {
+		ent = &memtrace_array[i];
+
+		/* We have onlined this chunk previously */
+		if (ent->nid == -1)
+			continue;
+
+		/* Remove from io mappings */
+		if (ent->mem) {
+			iounmap(ent->mem);
+			ent->mem = 0;
+		}
+
+		if (add_memory(ent->nid, ent->start, ent->size)) {
+			pr_err("Failed to add trace memory to node %d\n",
+				ent->nid);
+			ret += 1;
+			continue;
+		}
+
+		/*
+		 * If kernel isn't compiled with the auto online option
+		 * we need to online the memory ourselves.
+		 */
+		if (!memhp_auto_online) {
+			walk_memory_range(PFN_DOWN(ent->start),
+					  PFN_UP(ent->start + ent->size - 1),
+					  NULL, online_mem_block);
+		}
+
+		/*
+		 * Memory was added successfully so clean up references to it
+		 * so on reentry we can tell that this chunk was added.
+		 */
+		debugfs_remove_recursive(ent->dir);
+		pr_info("Added trace memory back to node %d\n", ent->nid);
+		ent->size = ent->start = ent->nid = -1;
+	}
+	if (ret)
+		return ret;
+
+	/* If all chunks of memory were added successfully, reset globals */
+	kfree(memtrace_array);
+	memtrace_array = NULL;
+	memtrace_size = 0;
+	memtrace_array_nr = 0;
+	return 0;
+}
+
 static int memtrace_enable_set(void *data, u64 val)
 {
-	if (memtrace_size)
+	u64 bytes;
+
+	/*
+	 * Don't attempt to do anything if size isn't aligned to a memory
+	 * block or equal to zero.
+	 */
+	bytes = memory_block_size_bytes();
+	if (val & (bytes - 1)) {
+		pr_err("Value must be aligned with 0x%llx\n", bytes);
 		return -EINVAL;
+	}
 
-	if (!val)
-		return -EINVAL;
+	/* Re-add/online previously removed/offlined memory */
+	if (memtrace_size) {
+		if (memtrace_online())
+			return -EAGAIN;
+	}
 
-	/* Make sure size is aligned to a memory block */
-	if (val & (memory_block_size_bytes() - 1))
-		return -EINVAL;
+	if (!val)
+		return 0;
 
+	/* Offline and remove memory */
 	if (memtrace_init_regions_runtime(val))
 		return -EINVAL;