[PATCH v3 5/5] mm,memory_hotplug: Allow userspace to enable/disable vmemmap

[Oscar Salvador <osalvador@suse.de>]

It seems that we have some users out there that want to expose all
hotpluggable memory to userspace, so this implements a toggling mechanism
for those users who want to disable it.

By default, vmemmap pages mechanism is enabled.

Signed-off-by: Oscar Salvador <osalvador@suse.de>
---
 drivers/base/memory.c          | 33 +++++++++++++++++++++++++++++++++
 include/linux/memory_hotplug.h |  3 +++
 mm/memory_hotplug.c            |  7 +++++++
 3 files changed, 43 insertions(+)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index d30d0f6c8ad0..5ec6b80de9dd 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -578,6 +578,35 @@ static DEVICE_ATTR_WO(soft_offline_page);
 static DEVICE_ATTR_WO(hard_offline_page);
 #endif
 
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+static ssize_t vmemmap_hotplug_show(struct device *dev,
+				    struct device_attribute *attr, char *buf)
+{
+	if (vmemmap_enabled)
+		return sprintf(buf, "enabled\n");
+	else
+		return sprintf(buf, "disabled\n");
+}
+
+static ssize_t vmemmap_hotplug_store(struct device *dev,
+			   struct device_attribute *attr,
+			   const char *buf, size_t count)
+{
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (sysfs_streq(buf, "enable"))
+		vmemmap_enabled = true;
+	else if (sysfs_streq(buf, "disable"))
+		vmemmap_enabled = false;
+	else
+		return -EINVAL;
+
+	return count;
+}
+static DEVICE_ATTR_RW(vmemmap_hotplug);
+#endif
+
 /*
  * Note that phys_device is optional.  It is here to allow for
  * differentiation between which *physical* devices each
@@ -794,6 +823,10 @@ static struct attribute *memory_root_attrs[] = {
 	&dev_attr_hard_offline_page.attr,
 #endif
 
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+	&dev_attr_vmemmap_hotplug.attr,
+#endif
+
 	&dev_attr_block_size_bytes.attr,
 	&dev_attr_auto_online_blocks.attr,
 	NULL
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index e1e8abf22a80..03d227d13301 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -134,6 +134,9 @@ extern int arch_add_memory(int nid, u64 start, u64 size,
 			struct mhp_restrictions *restrictions);
 extern u64 max_mem_size;
 
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+extern bool vmemmap_enabled;
+#endif
 extern bool memhp_auto_online;
 /* If movable_node boot option specified */
 extern bool movable_node_enabled;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 09d41339cd11..5ffe5375b87c 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -68,6 +68,10 @@ void put_online_mems(void)
 
 bool movable_node_enabled = false;
 
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+bool vmemmap_enabled __read_mostly = true;
+#endif
+
 #ifndef CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
 bool memhp_auto_online;
 #else
@@ -1108,6 +1112,9 @@ static unsigned long mhp_check_flags(unsigned long flags)
 	if (!flags)
 		return 0;
 
+	if (!vmemmap_enabled)
+		return 0;
+
 	if (flags != MHP_MEMMAP_ON_MEMORY) {
 		WARN(1, "Wrong flags value (%lx). Ignoring flags.\n", flags);
 		return 0;
-- 
2.12.3