[PATCH 0/2] docs: memory-hotplug: add details about locking internals

[PATCH 0/2] docs: memory-hotplug: add details about locking internals

[Mike Rapoport]

Hi,

As discussed at [1], the latest updates to memory hotplug documentation are
causing a conflict between docs and mmotm trees.
These patches resolve the conflict.

[1] https://lkml.org/lkml/2018/10/8/227

David Hildenbrand (1):
  docs/core-api: memory-hotplug: add some details about locking internals

Mike Rapoport (1):
  docs/core-api: rename memory-hotplug-notifier to memory-hotplug

 Documentation/core-api/index.rst                   |   2 +-
 Documentation/core-api/memory-hotplug-notifier.rst |  84 --------------
 Documentation/core-api/memory-hotplug.rst          | 125 +++++++++++++++++++++
 3 files changed, 126 insertions(+), 85 deletions(-)
 delete mode 100644 Documentation/core-api/memory-hotplug-notifier.rst
 create mode 100644 Documentation/core-api/memory-hotplug.rst

-- 
2.7.4

[PATCH 1/2] docs/core-api: rename memory-hotplug-notifier to memory-hotplug

[Mike Rapoport]

From: Mike Rapoport <rppt@linux.vnet.ibm.com>

to allow additions of new documentation about memory hotplug under the same
roof.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
 Documentation/core-api/index.rst                   |  2 +-
 Documentation/core-api/memory-hotplug-notifier.rst | 84 ---------------------
 Documentation/core-api/memory-hotplug.rst          | 87 ++++++++++++++++++++++
 3 files changed, 88 insertions(+), 85 deletions(-)
 delete mode 100644 Documentation/core-api/memory-hotplug-notifier.rst
 create mode 100644 Documentation/core-api/memory-hotplug.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 4f8a426..29c790f 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -32,7 +32,7 @@ Core utilities
    gfp_mask-from-fs-io
    timekeeping
    boot-time-mm
-   memory-hotplug-notifier
+   memory-hotplug
 
 
 Interfaces for kernel debugging
diff --git a/Documentation/core-api/memory-hotplug-notifier.rst b/Documentation/core-api/memory-hotplug-notifier.rst
deleted file mode 100644
index 35347cc..0000000
--- a/Documentation/core-api/memory-hotplug-notifier.rst
+++ /dev/null
@@ -1,84 +0,0 @@
-.. _memory_hotplug_notifier:
-
-=============================
-Memory hotplug event notifier
-=============================
-
-Hotplugging events are sent to a notification queue.
-
-There are six types of notification defined in ``include/linux/memory.h``:
-
-MEM_GOING_ONLINE
-  Generated before new memory becomes available in order to be able to
-  prepare subsystems to handle memory. The page allocator is still unable
-  to allocate from the new memory.
-
-MEM_CANCEL_ONLINE
-  Generated if MEM_GOING_ONLINE fails.
-
-MEM_ONLINE
-  Generated when memory has successfully brought online. The callback may
-  allocate pages from the new memory.
-
-MEM_GOING_OFFLINE
-  Generated to begin the process of offlining memory. Allocations are no
-  longer possible from the memory but some of the memory to be offlined
-  is still in use. The callback can be used to free memory known to a
-  subsystem from the indicated memory block.
-
-MEM_CANCEL_OFFLINE
-  Generated if MEM_GOING_OFFLINE fails. Memory is available again from
-  the memory block that we attempted to offline.
-
-MEM_OFFLINE
-  Generated after offlining memory is complete.
-
-A callback routine can be registered by calling::
-
-  hotplug_memory_notifier(callback_func, priority)
-
-Callback functions with higher values of priority are called before callback
-functions with lower values.
-
-A callback function must have the following prototype::
-
-  int callback_func(
-    struct notifier_block *self, unsigned long action, void *arg);
-
-The first argument of the callback function (self) is a pointer to the block
-of the notifier chain that points to the callback function itself.
-The second argument (action) is one of the event types described above.
-The third argument (arg) passes a pointer of struct memory_notify::
-
-	struct memory_notify {
-		unsigned long start_pfn;
-		unsigned long nr_pages;
-		int status_change_nid_normal;
-		int status_change_nid_high;
-		int status_change_nid;
-	}
-
-- start_pfn is start_pfn of online/offline memory.
-- nr_pages is # of pages of online/offline memory.
-- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
-  is (will be) set/clear, if this is -1, then nodemask status is not changed.
-- status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
-  is (will be) set/clear, if this is -1, then nodemask status is not changed.
-- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
-  set/clear. It means a new(memoryless) node gets new memory by online and a
-  node loses all memory. If this is -1, then nodemask status is not changed.
-
-  If status_changed_nid* >= 0, callback should create/discard structures for the
-  node if necessary.
-
-The callback routine shall return one of the values
-NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
-defined in ``include/linux/notifier.h``
-
-NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
-
-NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
-MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
-further processing of the notification queue.
-
-NOTIFY_STOP stops further processing of the notification queue.
diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
new file mode 100644
index 0000000..a99f2f2
--- /dev/null
+++ b/Documentation/core-api/memory-hotplug.rst
@@ -0,0 +1,87 @@
+.. _memory_hotplug:
+
+==============
+Memory hotplug
+==============
+
+Memory hotplug event notifier
+=============================
+
+Hotplugging events are sent to a notification queue.
+
+There are six types of notification defined in ``include/linux/memory.h``:
+
+MEM_GOING_ONLINE
+  Generated before new memory becomes available in order to be able to
+  prepare subsystems to handle memory. The page allocator is still unable
+  to allocate from the new memory.
+
+MEM_CANCEL_ONLINE
+  Generated if MEM_GOING_ONLINE fails.
+
+MEM_ONLINE
+  Generated when memory has successfully brought online. The callback may
+  allocate pages from the new memory.
+
+MEM_GOING_OFFLINE
+  Generated to begin the process of offlining memory. Allocations are no
+  longer possible from the memory but some of the memory to be offlined
+  is still in use. The callback can be used to free memory known to a
+  subsystem from the indicated memory block.
+
+MEM_CANCEL_OFFLINE
+  Generated if MEM_GOING_OFFLINE fails. Memory is available again from
+  the memory block that we attempted to offline.
+
+MEM_OFFLINE
+  Generated after offlining memory is complete.
+
+A callback routine can be registered by calling::
+
+  hotplug_memory_notifier(callback_func, priority)
+
+Callback functions with higher values of priority are called before callback
+functions with lower values.
+
+A callback function must have the following prototype::
+
+  int callback_func(
+    struct notifier_block *self, unsigned long action, void *arg);
+
+The first argument of the callback function (self) is a pointer to the block
+of the notifier chain that points to the callback function itself.
+The second argument (action) is one of the event types described above.
+The third argument (arg) passes a pointer of struct memory_notify::
+
+	struct memory_notify {
+		unsigned long start_pfn;
+		unsigned long nr_pages;
+		int status_change_nid_normal;
+		int status_change_nid_high;
+		int status_change_nid;
+	}
+
+- start_pfn is start_pfn of online/offline memory.
+- nr_pages is # of pages of online/offline memory.
+- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
+  is (will be) set/clear, if this is -1, then nodemask status is not changed.
+- status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
+  is (will be) set/clear, if this is -1, then nodemask status is not changed.
+- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
+  set/clear. It means a new(memoryless) node gets new memory by online and a
+  node loses all memory. If this is -1, then nodemask status is not changed.
+
+  If status_changed_nid* >= 0, callback should create/discard structures for the
+  node if necessary.
+
+The callback routine shall return one of the values
+NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
+defined in ``include/linux/notifier.h``
+
+NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
+
+NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
+MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
+further processing of the notification queue.
+
+NOTIFY_STOP stops further processing of the notification queue.
-- 
2.7.4

Re: [PATCH 1/2] docs/core-api: rename memory-hotplug-notifier to memory-hotplug

[David Hildenbrand]

On 11/10/2018 06:58, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.vnet.ibm.com>
> 
> to allow additions of new documentation about memory hotplug under the same
> roof.
> 
> Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
> ---
>  Documentation/core-api/index.rst                   |  2 +-
>  Documentation/core-api/memory-hotplug-notifier.rst | 84 ---------------------
>  Documentation/core-api/memory-hotplug.rst          | 87 ++++++++++++++++++++++
>  3 files changed, 88 insertions(+), 85 deletions(-)
>  delete mode 100644 Documentation/core-api/memory-hotplug-notifier.rst
>  create mode 100644 Documentation/core-api/memory-hotplug.rst
> 
> diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
> index 4f8a426..29c790f 100644
> --- a/Documentation/core-api/index.rst
> +++ b/Documentation/core-api/index.rst
> @@ -32,7 +32,7 @@ Core utilities
>     gfp_mask-from-fs-io
>     timekeeping
>     boot-time-mm
> -   memory-hotplug-notifier
> +   memory-hotplug
>  
>  
>  Interfaces for kernel debugging
> diff --git a/Documentation/core-api/memory-hotplug-notifier.rst b/Documentation/core-api/memory-hotplug-notifier.rst
> deleted file mode 100644
> index 35347cc..0000000
> --- a/Documentation/core-api/memory-hotplug-notifier.rst
> +++ /dev/null
> @@ -1,84 +0,0 @@
> -.. _memory_hotplug_notifier:
> -
> -=============================
> -Memory hotplug event notifier
> -=============================
> -
> -Hotplugging events are sent to a notification queue.
> -
> -There are six types of notification defined in ``include/linux/memory.h``:
> -
> -MEM_GOING_ONLINE
> -  Generated before new memory becomes available in order to be able to
> -  prepare subsystems to handle memory. The page allocator is still unable
> -  to allocate from the new memory.
> -
> -MEM_CANCEL_ONLINE
> -  Generated if MEM_GOING_ONLINE fails.
> -
> -MEM_ONLINE
> -  Generated when memory has successfully brought online. The callback may
> -  allocate pages from the new memory.
> -
> -MEM_GOING_OFFLINE
> -  Generated to begin the process of offlining memory. Allocations are no
> -  longer possible from the memory but some of the memory to be offlined
> -  is still in use. The callback can be used to free memory known to a
> -  subsystem from the indicated memory block.
> -
> -MEM_CANCEL_OFFLINE
> -  Generated if MEM_GOING_OFFLINE fails. Memory is available again from
> -  the memory block that we attempted to offline.
> -
> -MEM_OFFLINE
> -  Generated after offlining memory is complete.
> -
> -A callback routine can be registered by calling::
> -
> -  hotplug_memory_notifier(callback_func, priority)
> -
> -Callback functions with higher values of priority are called before callback
> -functions with lower values.
> -
> -A callback function must have the following prototype::
> -
> -  int callback_func(
> -    struct notifier_block *self, unsigned long action, void *arg);
> -
> -The first argument of the callback function (self) is a pointer to the block
> -of the notifier chain that points to the callback function itself.
> -The second argument (action) is one of the event types described above.
> -The third argument (arg) passes a pointer of struct memory_notify::
> -
> -	struct memory_notify {
> -		unsigned long start_pfn;
> -		unsigned long nr_pages;
> -		int status_change_nid_normal;
> -		int status_change_nid_high;
> -		int status_change_nid;
> -	}
> -
> -- start_pfn is start_pfn of online/offline memory.
> -- nr_pages is # of pages of online/offline memory.
> -- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
> -  is (will be) set/clear, if this is -1, then nodemask status is not changed.
> -- status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
> -  is (will be) set/clear, if this is -1, then nodemask status is not changed.
> -- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
> -  set/clear. It means a new(memoryless) node gets new memory by online and a
> -  node loses all memory. If this is -1, then nodemask status is not changed.
> -
> -  If status_changed_nid* >= 0, callback should create/discard structures for the
> -  node if necessary.
> -
> -The callback routine shall return one of the values
> -NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
> -defined in ``include/linux/notifier.h``
> -
> -NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
> -
> -NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
> -MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
> -further processing of the notification queue.
> -
> -NOTIFY_STOP stops further processing of the notification queue.
> diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
> new file mode 100644
> index 0000000..a99f2f2
> --- /dev/null
> +++ b/Documentation/core-api/memory-hotplug.rst
> @@ -0,0 +1,87 @@
> +.. _memory_hotplug:
> +
> +==============
> +Memory hotplug
> +==============
> +
> +Memory hotplug event notifier
> +=============================
> +
> +Hotplugging events are sent to a notification queue.
> +
> +There are six types of notification defined in ``include/linux/memory.h``:
> +
> +MEM_GOING_ONLINE
> +  Generated before new memory becomes available in order to be able to
> +  prepare subsystems to handle memory. The page allocator is still unable
> +  to allocate from the new memory.
> +
> +MEM_CANCEL_ONLINE
> +  Generated if MEM_GOING_ONLINE fails.
> +
> +MEM_ONLINE
> +  Generated when memory has successfully brought online. The callback may
> +  allocate pages from the new memory.
> +
> +MEM_GOING_OFFLINE
> +  Generated to begin the process of offlining memory. Allocations are no
> +  longer possible from the memory but some of the memory to be offlined
> +  is still in use. The callback can be used to free memory known to a
> +  subsystem from the indicated memory block.
> +
> +MEM_CANCEL_OFFLINE
> +  Generated if MEM_GOING_OFFLINE fails. Memory is available again from
> +  the memory block that we attempted to offline.
> +
> +MEM_OFFLINE
> +  Generated after offlining memory is complete.
> +
> +A callback routine can be registered by calling::
> +
> +  hotplug_memory_notifier(callback_func, priority)
> +
> +Callback functions with higher values of priority are called before callback
> +functions with lower values.
> +
> +A callback function must have the following prototype::
> +
> +  int callback_func(
> +    struct notifier_block *self, unsigned long action, void *arg);
> +
> +The first argument of the callback function (self) is a pointer to the block
> +of the notifier chain that points to the callback function itself.
> +The second argument (action) is one of the event types described above.
> +The third argument (arg) passes a pointer of struct memory_notify::
> +
> +	struct memory_notify {
> +		unsigned long start_pfn;
> +		unsigned long nr_pages;
> +		int status_change_nid_normal;
> +		int status_change_nid_high;
> +		int status_change_nid;
> +	}
> +
> +- start_pfn is start_pfn of online/offline memory.
> +- nr_pages is # of pages of online/offline memory.
> +- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
> +  is (will be) set/clear, if this is -1, then nodemask status is not changed.
> +- status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
> +  is (will be) set/clear, if this is -1, then nodemask status is not changed.
> +- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
> +  set/clear. It means a new(memoryless) node gets new memory by online and a
> +  node loses all memory. If this is -1, then nodemask status is not changed.
> +
> +  If status_changed_nid* >= 0, callback should create/discard structures for the
> +  node if necessary.
> +
> +The callback routine shall return one of the values
> +NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
> +defined in ``include/linux/notifier.h``
> +
> +NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
> +
> +NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
> +MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
> +further processing of the notification queue.
> +
> +NOTIFY_STOP stops further processing of the notification queue.
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 

Thanks,

David / dhildenb

[PATCH 2/2] docs/core-api: memory-hotplug: add some details about locking internals

[Mike Rapoport]

From: David Hildenbrand <david@redhat.com>

Let's document the magic a bit, especially why device_hotplug_lock is
required when adding/removing memory and how it all play together with
requests to online/offline memory from user space.

[ rppt: moved the text to Documentation/core-api/memory-hotplug.rst ]

Link: http://lkml.kernel.org/r/20180925091457.28651-7-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>
Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: John Allen <jallen@linux.vnet.ibm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Mathieu Malaterre <malat@debian.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michael Neuling <mikey@neuling.org>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
 Documentation/core-api/memory-hotplug.rst | 38 +++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
index a99f2f2..de7467e 100644
--- a/Documentation/core-api/memory-hotplug.rst
+++ b/Documentation/core-api/memory-hotplug.rst
@@ -85,3 +85,41 @@ MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
 further processing of the notification queue.
 
 NOTIFY_STOP stops further processing of the notification queue.
+
+Locking Internals
+=================
+
+When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
+the device_hotplug_lock should be held to:
+
+- synchronize against online/offline requests (e.g. via sysfs). This way, memory
+  block devices can only be accessed (.online/.state attributes) by user
+  space once memory has been fully added. And when removing memory, we
+  know nobody is in critical sections.
+- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
+
+Especially, there is a possible lock inversion that is avoided using
+device_hotplug_lock when adding memory and user space tries to online that
+memory faster than expected:
+
+- device_online() will first take the device_lock(), followed by
+  mem_hotplug_lock
+- add_memory_resource() will first take the mem_hotplug_lock, followed by
+  the device_lock() (while creating the devices, during bus_add_device()).
+
+As the device is visible to user space before taking the device_lock(), this
+can result in a lock inversion.
+
+onlining/offlining of memory should be done via device_online()/
+device_offline() - to make sure it is properly synchronized to actions
+via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
+
+When adding/removing/onlining/offlining memory or adding/removing
+heterogeneous/device memory, we should always hold the mem_hotplug_lock in
+write mode to serialise memory hotplug (e.g. access to global/zone
+variables).
+
+In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
+mode allows for a quite efficient get_online_mems/put_online_mems
+implementation, so code accessing memory can protect from that memory
+vanishing.
-- 
2.7.4

Re: [PATCH 2/2] docs/core-api: memory-hotplug: add some details about locking internals

[David Hildenbrand]

On 11/10/2018 06:58, Mike Rapoport wrote:
> From: David Hildenbrand <david@redhat.com>
> 
> Let's document the magic a bit, especially why device_hotplug_lock is
> required when adding/removing memory and how it all play together with
> requests to online/offline memory from user space.
> 
> [ rppt: moved the text to Documentation/core-api/memory-hotplug.rst ]
> 
> Link: http://lkml.kernel.org/r/20180925091457.28651-7-david@redhat.com
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>
> Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Balbir Singh <bsingharora@gmail.com>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: John Allen <jallen@linux.vnet.ibm.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: Len Brown <lenb@kernel.org>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Mathieu Malaterre <malat@debian.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Michael Neuling <mikey@neuling.org>
> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Philippe Ombredanne <pombredanne@nexb.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
>  Documentation/core-api/memory-hotplug.rst | 38 +++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
> index a99f2f2..de7467e 100644
> --- a/Documentation/core-api/memory-hotplug.rst
> +++ b/Documentation/core-api/memory-hotplug.rst
> @@ -85,3 +85,41 @@ MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
>  further processing of the notification queue.
>  
>  NOTIFY_STOP stops further processing of the notification queue.
> +
> +Locking Internals
> +=================
> +
> +When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
> +the device_hotplug_lock should be held to:
> +
> +- synchronize against online/offline requests (e.g. via sysfs). This way, memory
> +  block devices can only be accessed (.online/.state attributes) by user
> +  space once memory has been fully added. And when removing memory, we
> +  know nobody is in critical sections.
> +- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
> +
> +Especially, there is a possible lock inversion that is avoided using
> +device_hotplug_lock when adding memory and user space tries to online that
> +memory faster than expected:
> +
> +- device_online() will first take the device_lock(), followed by
> +  mem_hotplug_lock
> +- add_memory_resource() will first take the mem_hotplug_lock, followed by
> +  the device_lock() (while creating the devices, during bus_add_device()).
> +
> +As the device is visible to user space before taking the device_lock(), this
> +can result in a lock inversion.
> +
> +onlining/offlining of memory should be done via device_online()/
> +device_offline() - to make sure it is properly synchronized to actions
> +via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
> +
> +When adding/removing/onlining/offlining memory or adding/removing
> +heterogeneous/device memory, we should always hold the mem_hotplug_lock in
> +write mode to serialise memory hotplug (e.g. access to global/zone
> +variables).
> +
> +In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
> +mode allows for a quite efficient get_online_mems/put_online_mems
> +implementation, so code accessing memory can protect from that memory
> +vanishing.
> 

Looks good to me.

-- 

Thanks,

David / dhildenb

Re: [PATCH 0/2] docs: memory-hotplug: add details about locking internals

[Jonathan Corbet]

On Thu, 11 Oct 2018 07:58:15 +0300
Mike Rapoport <rppt@linux.ibm.com> wrote:

> As discussed at [1], the latest updates to memory hotplug documentation are
> causing a conflict between docs and mmotm trees.
> These patches resolve the conflict.
> 
> [1] https://lkml.org/lkml/2018/10/8/227
> 
> David Hildenbrand (1):
>   docs/core-api: memory-hotplug: add some details about locking internals
> 
> Mike Rapoport (1):
>   docs/core-api: rename memory-hotplug-notifier to memory-hotplug

I've applied the pair, thanks.

jon

Re: [PATCH 0/2] docs: memory-hotplug: add details about locking internals

[David Hildenbrand]

On 12.10.18 19:21, Jonathan Corbet wrote:
> On Thu, 11 Oct 2018 07:58:15 +0300
> Mike Rapoport <rppt@linux.ibm.com> wrote:
> 
>> As discussed at [1], the latest updates to memory hotplug documentation are
>> causing a conflict between docs and mmotm trees.
>> These patches resolve the conflict.
>>
>> [1] https://lkml.org/lkml/2018/10/8/227
>>
>> David Hildenbrand (1):
>>   docs/core-api: memory-hotplug: add some details about locking internals
>>
>> Mike Rapoport (1):
>>   docs/core-api: rename memory-hotplug-notifier to memory-hotplug
> 
> I've applied the pair, thanks.
> 
> jon
> 

Looking at linux-next, we now have duplicate documentation:

$ git grep mem_hotplug_lock
Documentation/admin-guide/mm/memory-hotplug.rst:  mem_hotplug_lock
Documentation/admin-guide/mm/memory-hotplug.rst:- add_memory_resource()
will first take the mem_hotplug_lock, followed by
Documentation/admin-guide/mm/memory-hotplug.rst:heterogeneous/device
memory, we should always hold the mem_hotplug_lock in
Documentation/admin-guide/mm/memory-hotplug.rst:In addition,
mem_hotplug_lock (in contrast to device_hotplug_lock) in read

Documentation/core-api/memory-hotplug.rst:  mem_hotplug_lock
Documentation/core-api/memory-hotplug.rst:- add_memory_resource() will
first take the mem_hotplug_lock, followed by
Documentation/core-api/memory-hotplug.rst:heterogeneous/device memory,
we should always hold the mem_hotplug_lock in
Documentation/core-api/memory-hotplug.rst:In addition, mem_hotplug_lock
(in contrast to device_hotplug_lock) in read

It really only should go to Documentation/core-api/memory-hotplug.rst

Should I send a patch or who can fix that up?

-- 

Thanks,

David / dhildenb