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[Xen-devel] [PATCH v5 1/2] memory-hotplug: add automatic onlining policy for the newly added memory

Currently, all newly added memory blocks remain in 'offline' state unless
someone onlines them, some linux distributions carry special udev rules

SUBSYSTEM=="memory", ACTION=="add", ATTR{state}=="offline", ATTR{state}="online"

to make this happen automatically. This is not a great solution for virtual
machines where memory hotplug is being used to address high memory pressure
situations as such onlining is slow and a userspace process doing this
(udev) has a chance of being killed by the OOM killer as it will probably
require to allocate some memory.

Introduce default policy for the newly added memory blocks in
/sys/devices/system/memory/auto_online_blocks file with two possible
values: "offline" which preserves the current behavior and "online" which
causes all newly added memory blocks to go online as soon as they're added.
The default is "offline".

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Changes since v4:
- Use memory_block_change_state() through walk_memory_range() instead of
  online_pages() to correctly handle possible failures [David Rientjes]
- Minor memory-hotplug.txt changes (keep the old title, explicitly word
  that we have a global policy here) [David Rientjes, Daniel Kiper]
 Documentation/memory-hotplug.txt | 20 +++++++++++++++++---
 drivers/base/memory.c            | 34 +++++++++++++++++++++++++++++++++-
 drivers/xen/balloon.c            |  2 +-
 include/linux/memory.h           |  3 +++
 include/linux/memory_hotplug.h   |  4 +++-
 mm/memory_hotplug.c              | 17 +++++++++++++++--
 6 files changed, 72 insertions(+), 8 deletions(-)

diff --git a/Documentation/memory-hotplug.txt b/Documentation/memory-hotplug.txt
index ce2cfcf..b259c6c 100644
--- a/Documentation/memory-hotplug.txt
+++ b/Documentation/memory-hotplug.txt
@@ -256,10 +256,24 @@ If the memory block is offline, you'll read "offline".
 5.2. How to online memory
-Even if the memory is hot-added, it is not at ready-to-use state.
-For using newly added memory, you have to "online" the memory block.
+When the memory is hot-added, the kernel decides whether or not to "online"
+it according to the policy which can be read from "auto_online_blocks" file:
-For onlining, you have to write "online" to the memory block's state file as:
+% cat /sys/devices/system/memory/auto_online_blocks
+The default is "offline" which means the newly added memory is not in a
+ready-to-use state and you have to "online" the newly added memory blocks
+manually. Automatic onlining can be requested by writing "online" to
+"auto_online_blocks" file:
+% echo online > /sys/devices/system/memory/auto_online_blocks
+This sets a global policy and impacts all memory blocks that will subsequently
+be hotplugged. Currently offline blocks keep their state.
+If the automatic onlining wasn't requested or some memory block was offlined
+it is possible to change the individual block's state by writing to the "state"
 % echo online > /sys/devices/system/memory/memoryXXX/state
diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 25425d3..4fc240e 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -251,7 +251,7 @@ memory_block_action(unsigned long phys_index, unsigned long 
action, int online_t
        return ret;
-static int memory_block_change_state(struct memory_block *mem,
+int memory_block_change_state(struct memory_block *mem,
                unsigned long to_state, unsigned long from_state_req)
        int ret = 0;
@@ -439,6 +439,37 @@ print_block_size(struct device *dev, struct 
device_attribute *attr,
 static DEVICE_ATTR(block_size_bytes, 0444, print_block_size, NULL);
+ * Memory auto online policy.
+ */
+static ssize_t
+show_auto_online_blocks(struct device *dev, struct device_attribute *attr,
+                       char *buf)
+       if (memhp_auto_online)
+               return sprintf(buf, "online\n");
+       else
+               return sprintf(buf, "offline\n");
+static ssize_t
+store_auto_online_blocks(struct device *dev, struct device_attribute *attr,
+                        const char *buf, size_t count)
+       if (sysfs_streq(buf, "online"))
+               memhp_auto_online = true;
+       else if (sysfs_streq(buf, "offline"))
+               memhp_auto_online = false;
+       else
+               return -EINVAL;
+       return count;
+static DEVICE_ATTR(auto_online_blocks, 0644, show_auto_online_blocks,
+                  store_auto_online_blocks);
  * Some architectures will have custom drivers to do this, and
  * will not need to do it from userspace.  The fake hot-add code
  * as well as ppc64 will do all of their discovery in userspace
@@ -737,6 +768,7 @@ static struct attribute *memory_root_attrs[] = {
+       &dev_attr_auto_online_blocks.attr,
diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 12eab50..890c3b5 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -338,7 +338,7 @@ static enum bp_state reserve_additional_memory(void)
-       rc = add_memory_resource(nid, resource);
+       rc = add_memory_resource(nid, resource, false);
        if (rc) {
                pr_warn("Cannot add additional memory (%i)\n", rc);
                goto err;
diff --git a/include/linux/memory.h b/include/linux/memory.h
index 8b8d8d1..82730ad 100644
--- a/include/linux/memory.h
+++ b/include/linux/memory.h
@@ -109,6 +109,9 @@ extern void unregister_memory_notifier(struct 
notifier_block *nb);
 extern int register_memory_isolate_notifier(struct notifier_block *nb);
 extern void unregister_memory_isolate_notifier(struct notifier_block *nb);
 extern int register_new_memory(int, struct mem_section *);
+extern int memory_block_change_state(struct memory_block *mem,
+                                    unsigned long to_state,
+                                    unsigned long from_state_req);
 extern int unregister_memory_section(struct mem_section *);
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 2ea574f..4b7949a 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -99,6 +99,8 @@ extern void __online_page_free(struct page *page);
 extern int try_online_node(int nid);
+extern bool memhp_auto_online;
 extern bool is_pageblock_removable_nolock(struct page *page);
 extern int arch_remove_memory(u64 start, u64 size);
@@ -267,7 +269,7 @@ static inline void remove_memory(int nid, u64 start, u64 
size) {}
 extern int walk_memory_range(unsigned long start_pfn, unsigned long end_pfn,
                void *arg, int (*func)(struct memory_block *, void *));
 extern int add_memory(int nid, u64 start, u64 size);
-extern int add_memory_resource(int nid, struct resource *resource);
+extern int add_memory_resource(int nid, struct resource *resource, bool 
 extern int zone_for_memory(int nid, u64 start, u64 size, int zone_default,
                bool for_device);
 extern int arch_add_memory(int nid, u64 start, u64 size, bool for_device);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index a042a9d..43eb230 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -76,6 +76,9 @@ static struct {
 #define memhp_lock_acquire()      lock_map_acquire(&mem_hotplug.dep_map)
 #define memhp_lock_release()      lock_map_release(&mem_hotplug.dep_map)
+bool memhp_auto_online;
 void get_online_mems(void)
@@ -1231,8 +1234,13 @@ int zone_for_memory(int nid, u64 start, u64 size, int 
        return zone_default;
+static int online_memory_block(struct memory_block *mem, void *arg)
+       return memory_block_change_state(mem, MEM_ONLINE, MEM_OFFLINE);
 /* we are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG */
-int __ref add_memory_resource(int nid, struct resource *res)
+int __ref add_memory_resource(int nid, struct resource *res, bool online)
        u64 start, size;
        pg_data_t *pgdat = NULL;
@@ -1292,6 +1300,11 @@ int __ref add_memory_resource(int nid, struct resource 
        /* create new memmap entry */
        firmware_map_add_hotplug(start, start + size, "System RAM");
+       /* online pages if requested */
+       if (online)
+               walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1),
+                                 NULL, online_memory_block);
        goto out;
@@ -1315,7 +1328,7 @@ int __ref add_memory(int nid, u64 start, u64 size)
        if (!res)
                return -EEXIST;
-       ret = add_memory_resource(nid, res);
+       ret = add_memory_resource(nid, res, memhp_auto_online);
        if (ret < 0)
        return ret;

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