From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 9BA2B6B6B13 for ; Mon, 3 Dec 2018 16:06:39 -0500 (EST) Received: by mail-ed1-f72.google.com with SMTP id d41so6990284eda.12 for ; Mon, 03 Dec 2018 13:06:39 -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 e44sor8107353ede.13.2018.12.03.13.06.38 for (Google Transport Security); Mon, 03 Dec 2018 13:06:38 -0800 (PST) Date: Mon, 3 Dec 2018 21:06:36 +0000 From: Wei Yang Subject: Re: [PATCH v3 1/2] mm, sparse: drop pgdat_resize_lock in sparse_add/remove_one_section() Message-ID: <20181203210636.cdocbv7432dqjl7z@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 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