From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f198.google.com (mail-qt1-f198.google.com [209.85.160.198]) by kanga.kvack.org (Postfix) with ESMTP id 3CC116B576F for ; Fri, 30 Nov 2018 04:19:17 -0500 (EST) Received: by mail-qt1-f198.google.com with SMTP id z6so4728354qtj.21 for ; Fri, 30 Nov 2018 01:19:17 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id q8si303801qvr.202.2018.11.30.01.19.16 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 30 Nov 2018 01:19:16 -0800 (PST) Subject: Re: [PATCH v3 1/2] mm, sparse: drop pgdat_resize_lock in sparse_add/remove_one_section() References: <20181128091243.19249-1-richard.weiyang@gmail.com> <20181129155316.8174-1-richard.weiyang@gmail.com> <20181130042815.t44nroyqcqa3tpgv@master> From: David Hildenbrand Message-ID: Date: Fri, 30 Nov 2018 10:19:13 +0100 MIME-Version: 1.0 In-Reply-To: <20181130042815.t44nroyqcqa3tpgv@master> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Wei Yang Cc: mhocko@suse.com, dave.hansen@intel.com, osalvador@suse.de, akpm@linux-foundation.org, linux-mm@kvack.org >> 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, David / dhildenb