On Mon, Dec 03, 2018 at 12:25:20PM +0100, David Hildenbrand wrote: >On 30.11.18 05:28, Wei Yang wrote: >> 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 > >BTW, it really should go into > >Documentation/core-api/memory-hotplug.rst > >Something got wrong while merging this in linux-next, so now we have >duplicate documentation and the one in >Documentation/admin-guide/mm/memory-hotplug.rst about locking internals >has to go. > Sounds reasonable. Admin may not necessary need to understand the internal locking. -- Wei Yang Help you, Help me
