[PATCH RFC] memory-hotplug: add automatic onlining policy for the newly added memory

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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

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/hotplug_autoonline file with two possible
values: "offline" (the default) which preserves the current behavior and
"online" which causes all newly added memory blocks to go online as
soon as they're added.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Daniel Kiper <daniel.kiper@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Gu Zheng <guz.fnst@cn.fujitsu.com>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
- I was able to find previous attempts to fix the issue, e.g.:
  http://marc.info/?l=linux-kernel&m=137425951924598&w=2
  http://marc.info/?l=linux-acpi&m=127186488905382
  but I'm not completely sure why it didn't work out and the solution
  I suggest is not 'smart enough', thus 'RFC'.
---
 Documentation/memory-hotplug.txt | 21 +++++++++++++++++----
 drivers/base/memory.c            | 35 +++++++++++++++++++++++++++++++++++
 include/linux/memory_hotplug.h   |  2 ++
 mm/memory_hotplug.c              |  8 ++++++++
 4 files changed, 62 insertions(+), 4 deletions(-)

diff --git a/Documentation/memory-hotplug.txt b/Documentation/memory-hotplug.txt
index ce2cfcf..fe576d9 100644
--- a/Documentation/memory-hotplug.txt
+++ b/Documentation/memory-hotplug.txt
@@ -254,12 +254,25 @@ If the memory block is online, you'll read "online".
 If the memory block is offline, you'll read "offline".
 
 
-5.2. How to online memory
+5.2. Memory onlining
 ------------
-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 "hotplug_autoonline" file:
 
-For onlining, you have to write "online" to the memory block's state file as:
+% cat /sys/devices/system/memory/hotplug_autoonline
+
+The default is "offline" which means the newly added memory will not be at
+ready-to-use state and you have to "online" the newly added memory blocks
+manually.
+
+Automatic onlining can be requested by writing "online" to "hotplug_autoonline"
+file:
+
+% echo online > /sys/devices/system/memory/hotplug_autoonline
+
+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"
+file:
 
 % echo online > /sys/devices/system/memory/memoryXXX/state
 
diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 25425d3..001fefe 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -438,6 +438,40 @@ 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_memhp_autoonline(struct device *dev, struct device_attribute *attr,
+		      char *buf)
+{
+	if (memhp_autoonline == MMOP_ONLINE_KEEP)
+		return sprintf(buf, "online\n");
+	else if (memhp_autoonline == MMOP_OFFLINE)
+		return sprintf(buf, "offline\n");
+	else
+		return sprintf(buf, "unknown\n");
+}
+
+static ssize_t
+store_memhp_autoonline(struct device *dev, struct device_attribute *attr,
+		       const char *buf, size_t count)
+{
+	if (sysfs_streq(buf, "online"))
+		memhp_autoonline = MMOP_ONLINE_KEEP;
+	else if (sysfs_streq(buf, "offline"))
+		memhp_autoonline = MMOP_OFFLINE;
+	else
+		return -EINVAL;
+
+	return count;
+}
+
+static DEVICE_ATTR(hotplug_autoonline, 0644, show_memhp_autoonline,
+		   store_memhp_autoonline);
+
 /*
  * Some architectures will have custom drivers to do this, and
  * will not need to do it from userspace.  The fake hot-add code
@@ -737,6 +771,7 @@ static struct attribute *memory_root_attrs[] = {
 #endif
 
 	&dev_attr_block_size_bytes.attr,
+	&dev_attr_hotplug_autoonline.attr,
 	NULL
 };
 
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 2ea574f..fb64eea 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 int memhp_autoonline;
+
 #ifdef CONFIG_MEMORY_HOTREMOVE
 extern bool is_pageblock_removable_nolock(struct page *page);
 extern int arch_remove_memory(u64 start, u64 size);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 67d488a..37a190e 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)
 
+int memhp_autoonline = MMOP_OFFLINE;
+EXPORT_SYMBOL_GPL(memhp_autoonline);
+
 void get_online_mems(void)
 {
 	might_sleep();
@@ -1292,6 +1295,11 @@ int __ref add_memory_resource(int nid, struct resource *res)
 	/* create new memmap entry */
 	firmware_map_add_hotplug(start, start + size, "System RAM");
 
+	/* online the region if it is requested by current policy */
+	if (memhp_autoonline != MMOP_OFFLINE)
+		online_pages(start >> PAGE_SHIFT, size >> PAGE_SHIFT,
+			     memhp_autoonline);
+
 	goto out;
 
 error:
-- 
2.4.3

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