Re: [PATCH v3 1/2] mm, sparse: drop pgdat_resize_lock in sparse_add/remove_one_section()

[Wei Yang <richard.weiyang@gmail.com>]

On Fri, Nov 30, 2018 at 10:19:13AM +0100, David Hildenbrand wrote:
>>> I suggest adding what you just found out to
>>> Documentation/admin-guide/mm/memory-hotplug.rst "Locking Internals".
>>> Maybe a new subsection for mem_hotplug_lock. And eventually also
>>> pgdat_resize_lock.
>> 
>> Well, I am not good at document writting. Below is my first trial.  Look
>> forward your comments.
>
>I'll have a look, maybe also Oscar and Michal can have a look. I guess
>we don't have to cover all now, we can add more details as we discover them.
>
>> 
>> BTW, in case I would send a new version with this, would I put this into
>> a separate one or merge this into current one?
>
>I would put this into a separate patch.
>
>> 
>> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
>> index 5c4432c96c4b..1548820a0762 100644
>> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
>> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
>> @@ -396,6 +396,20 @@ Need more implementation yet....
>>  Locking Internals
>>  =================
>>  
>> +There are three locks involved in memory-hotplug, two global lock and one local
>> +lock:
>> +
>> +- device_hotplug_lock
>> +- mem_hotplug_lock
>> +- device_lock
>> +
>> +Currently, they are twisted together for all kinds of reasons. The following
>> +part is divded into device_hotplug_lock and mem_hotplug_lock parts
>
>s/divded/divided/
>
>> +respectively to describe those tricky situations.
>> +
>> +device_hotplug_lock
>> +---------------------
>> +
>>  When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
>>  the device_hotplug_lock should be held to:
>>  
>> @@ -417,14 +431,21 @@ memory faster than expected:
>>  As the device is visible to user space before taking the device_lock(), this
>>  can result in a lock inversion.
>>  
>> +mem_hotplug_lock
>> +---------------------
>> +
>
>I would this section start after the following paragraph, as most of
>that paragraph belongs to the device_hotplug_lock.
>
>
>>  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)
>> +device_offline() - to make sure it is properly synchronized to actions via
>> +sysfs. Even mem_hotplug_lock is used to protect the process, because of the
>> +lock inversion described above, holding device_hotplug_lock is still 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).
>> +variables). Currently, we take advantage of this to serialise sparsemem's
>> +mem_section handling in sparse_add_one_section() and
>> +sparse_remove_one_section().
>>  
>>  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
>> 
>>>
>>>
>>> Thanks,
>>>
>>> David / dhildenb
>> 
>
>Apart from that looks good to me, thanks!
>

Thanks :-)

>
>-- 
>
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me