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

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

On Thu, Nov 29, 2018 at 05:06:15PM +0100, David Hildenbrand wrote:
>On 29.11.18 16:53, Wei Yang wrote:
>> pgdat_resize_lock is used to protect pgdat's memory region information
>> like: node_start_pfn, node_present_pages, etc. While in function
>> sparse_add/remove_one_section(), pgdat_resize_lock is used to protect
>> initialization/release of one mem_section. This looks not proper.
>> 
>> Based on current implementation, even remove this lock, mem_section
>> is still away from contention, because it is protected by global
>> mem_hotpulg_lock.
>
>s/mem_hotpulg_lock/mem_hotplug_lock/
>
>> 
>> Following is the current call trace of sparse_add/remove_one_section()
>> 
>>     mem_hotplug_begin()
>>     arch_add_memory()
>>        add_pages()
>>            __add_pages()
>>                __add_section()
>>                    sparse_add_one_section()
>>     mem_hotplug_done()
>> 
>>     mem_hotplug_begin()
>>     arch_remove_memory()
>>         __remove_pages()
>>             __remove_section()
>>                 sparse_remove_one_section()
>>     mem_hotplug_done()
>> 
>> The comment above the pgdat_resize_lock also mentions "Holding this will
>> also guarantee that any pfn_valid() stays that way.", which is true with
>> the current implementation and false after this patch. But current
>> implementation doesn't meet this comment. There isn't any pfn walkers
>> to take the lock so this looks like a relict from the past. This patch
>> also removes this comment.
>
>Should we start to document which lock is expected to protect what?
>
>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.

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?

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
+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
+---------------------
+
 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

-- 
Wei Yang
Help you, Help me