From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id A07546B5B01 for ; Fri, 30 Nov 2018 19:31:50 -0500 (EST) Received: by mail-ed1-f69.google.com with SMTP id b3so3658183edi.0 for ; Fri, 30 Nov 2018 16:31:50 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id w19-v6sor2165787ejv.42.2018.11.30.16.31.49 for (Google Transport Security); Fri, 30 Nov 2018 16:31:49 -0800 (PST) Date: Sat, 1 Dec 2018 00:31:47 +0000 From: Wei Yang Subject: Re: [PATCH v3 1/2] mm, sparse: drop pgdat_resize_lock in sparse_add/remove_one_section() Message-ID: <20181201003147.ahb6nte5bxrotmhx@master> Reply-To: Wei Yang References: <20181128091243.19249-1-richard.weiyang@gmail.com> <20181129155316.8174-1-richard.weiyang@gmail.com> <20181130042815.t44nroyqcqa3tpgv@master> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: Wei Yang , mhocko@suse.com, dave.hansen@intel.com, osalvador@suse.de, akpm@linux-foundation.org, linux-mm@kvack.org 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