Subject: Re: linux-next: manual merge of the akpm tree with the jc_docs tree

From: Mike Rapoport <[email-masked]>
Date: Mon, 8 Oct 2018 11:13:16 +0300

On Mon, Oct 08, 2018 at 06:38:40PM +1100, Stephen Rothwell wrote:
> Hi all,
> 
> Today's linux-next merge of the akpm tree got conflicts in:
> 
>   Documentation/memory-hotplug.txt
>   Documentation/admin-guide/mm/memory-hotplug.rst
> 
> between commits:
> 
>   6bf53999a3a2 ("docs: move memory hotplug description into admin-guide/mm")
>   98cee6742c80 ("docs/vm: split memory hotplug notifier description to Documentation/core-api")
> 
> from the jc_docs tree and patch:
> 
>   "memory-hotplug.txt: add some details about locking internals"
> 
> from the akpm tree.
> 
> I fixed it up (see below) and can carry the fix as necessary. This
> is now fixed as far as linux-next is concerned, but any non trivial
> conflicts should be mentioned to your upstream maintainer when your tree
> is submitted for merging.  You may also want to consider cooperating
> with the maintainer of the conflicting tree to minimise any particularly
> complex conflicts.

Somehow I've managed to miss the patches from David :(

I should have called the new
Documentation/core-api/memory-hotplug-notifier.rst memory-hotplug.rst and
then the details about locking would fit there just fine.

I can do the renaming and add the patch

  "memory-hotplug.txt: add some details about locking internals"

on top of the jc_docs tree.

Does it sound Ok?

> -- 
> Cheers,
> Stephen Rothwell
> 
> diff --cc Documentation/admin-guide/mm/memory-hotplug.rst
> index 0b9c83effaa4,ce4faa5530fa..000000000000
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@@ -413,6 -413,128 +413,46 @@@ Need more implementation yet...
>    - Notification completion of remove works by OS to firmware.
>    - Guard from remove if not yet.
>   
>  -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 MEMORY_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 MEMORY_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.
>  -
> + 
> + 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.
> + 
> + 
>   Future Work
>   ===========
>   

-- 
Sincerely yours,
Mike.