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
