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From: David Hildenbrand <david@redhat.com>
To: Oscar Salvador <osalvador@techadventures.net>,
	Michal Hocko <mhocko@kernel.org>
Cc: pavel.tatashin@microsoft.com, linux-nvdimm@lists.01.org,
	dave.hansen@intel.com, linux-kernel@vger.kernel.org,
	mingo@kernel.org, linux-mm@kvack.org, jglisse@redhat.com,
	rppt@linux.vnet.ibm.com, kirill.shutemov@linux.intel.com,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	akpm@linux-foundation.org
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap
Date: Thu, 27 Sep 2018 17:41:19 +0200	[thread overview]
Message-ID: <27f1265b-a8b4-b203-077b-468d1e823cc8@redhat.com> (raw)
In-Reply-To: <20180927145035.GA21373@techadventures.net>

On 27/09/2018 16:50, Oscar Salvador wrote:
> On Thu, Sep 27, 2018 at 03:13:29PM +0200, Michal Hocko wrote:
>> On Thu 27-09-18 14:25:37, Oscar Salvador wrote:
>>> On Thu, Sep 27, 2018 at 01:09:26PM +0200, Michal Hocko wrote:
>>>>> So there were a few things I wasn't sure we could pull outside of the
>>>>> hotplug lock. One specific example is the bits related to resizing the pgdat
>>>>> and zone. I wanted to avoid pulling those bits outside of the hotplug lock.
>>>>
>>>> Why would that be a problem. There are dedicated locks for resizing.
>>>
>>> True is that move_pfn_range_to_zone() manages the locks for pgdat/zone resizing,
>>> but it also takes care of calling init_currently_empty_zone() in case the zone is empty.
>>> Could not that be a problem if we take move_pfn_range_to_zone() out of the lock?
>>
>> I would have to double check but is the hotplug lock really serializing
>> access to the state initialized by init_currently_empty_zone? E.g.
>> zone_start_pfn is a nice example of a state that is used outside of the
>> lock. zone's free lists are similar. So do we really need the hoptlug
>> lock? And more broadly, what does the hotplug lock is supposed to
>> serialize in general. A proper documentation would surely help to answer
>> these questions. There is way too much of "do not touch this code and
>> just make my particular hack" mindset which made the whole memory
>> hotplug a giant pile of mess. We really should start with some proper
>> engineering here finally.
> 
> CC David
> 
> David has been looking into this lately, he even has updated memory-hotplug.txt
> with some more documentation about the locking aspect [1].
> And with this change [2], the hotplug lock has been moved
> to the online/offline_pages.
> 
> From what I see (I might be wrong), the hotplug lock is there
> to serialize the online/offline operations.

mem_hotplug_lock is especially relevant for users of
get_online_mems/put_online_mems. Whatever affects them, you can't move
out of the lock.

Everything else is theoretically serialized via device_hotplug_lock now.

> 
> In online_pages, we do (among other things):
> 
> a) initialize the zone and its pages, and link them to the zone
> b) re-adjust zone/pgdat nr of pages (present, spanned, managed)
> b) check if the node changes in regard of N_MEMORY, N_HIGH_MEMORY or N_NORMAL_MEMORY.
> c) fire notifiers
> d) rebuild the zonelists in case we got a new zone
> e) online memory sections and free the pages to the buddy allocator
> f) wake up kswapd/kcompactd in case we got a new node
> 
> while in offline_pages we do the opposite.
> 
> Hotplug lock here serializes the operations as a whole, online and offline memory,
> so they do not step on each other's feet.
> 
> Having said that, we might be able to move some of those operations out of the hotplug lock.
> The device_hotplug_lock coming from every memblock (which is taken in device_online/device_offline) should protect
> us against some operations being made on the same memblock (e.g: touching the same pages).

Yes, very right.


-- 

Thanks,

David / dhildenb
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  reply	other threads:[~2018-09-27 15:41 UTC|newest]

Thread overview: 77+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-25 20:18 [PATCH v5 0/4] Address issues slowing persistent memory initialization Alexander Duyck
2018-09-25 20:19 ` [PATCH v5 1/4] mm: Remove now defunct NO_BOOTMEM from depends list for deferred init Alexander Duyck
2018-09-25 21:05   ` Mike Rapoport
2018-09-25 20:20 ` [PATCH v5 2/4] mm: Provide kernel parameter to allow disabling page init poisoning Alexander Duyck
2018-09-25 20:26   ` Dave Hansen
2018-09-25 20:38     ` Alexander Duyck
2018-09-25 22:14       ` Dave Hansen
2018-09-25 22:27         ` Alexander Duyck
2018-09-26  7:38   ` Michal Hocko
2018-09-26 15:24     ` Alexander Duyck
2018-09-26 15:39       ` Michal Hocko
2018-09-26 15:41       ` Dave Hansen
2018-09-26 16:18         ` Alexander Duyck
2018-09-26 15:36     ` Dave Hansen
2018-09-26 22:36       ` Andrew Morton
2018-09-25 20:20 ` [PATCH v5 3/4] mm: Create non-atomic version of SetPageReserved for init use Alexander Duyck
2018-09-25 20:21 ` [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Alexander Duyck
2018-09-26  7:55   ` Michal Hocko
2018-09-26 18:25     ` Alexander Duyck
2018-09-26 18:52       ` Dan Williams
2018-09-27 11:20         ` Michal Hocko
2018-09-27 11:09       ` Michal Hocko
2018-09-27 12:25         ` Oscar Salvador
2018-09-27 13:13           ` Michal Hocko
2018-09-27 14:50             ` Oscar Salvador
2018-09-27 15:41               ` David Hildenbrand [this message]
2018-09-28  8:12             ` Oscar Salvador
2018-09-28  8:44               ` Oscar Salvador
2018-09-28 15:50                 ` Dan Williams
2018-09-27 12:32       ` Oscar Salvador
2018-10-08 21:01   ` Dan Williams
2018-10-08 21:38     ` Alexander Duyck
2018-10-08 22:00       ` Dan Williams
2018-10-08 22:07         ` Alexander Duyck
2018-10-08 22:36         ` Alexander Duyck
2018-10-08 22:59           ` Dan Williams
2018-10-08 23:34     ` [mm PATCH] memremap: Fix reference count for pgmap in devm_memremap_pages Alexander Duyck
2018-10-09  0:20       ` Dan Williams
2018-10-09 17:00   ` [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Yi Zhang
2018-10-09 18:04     ` Dan Williams
2018-10-09 20:26       ` Alexander Duyck
2018-10-09 21:19         ` Dan Williams
2018-10-10 12:52           ` Yi Zhang
2018-10-10 15:27             ` Alexander Duyck
2018-10-11  8:17               ` Yi Zhang
2018-10-10  9:58         ` Michal Hocko
2018-10-10 16:39           ` Alexander Duyck
2018-10-10 17:24             ` Michal Hocko
2018-10-10 17:39               ` Alexander Duyck
2018-10-10 17:53                 ` Michal Hocko
2018-10-10 18:13                   ` Alexander Duyck
2018-10-10 18:52                 ` Michal Hocko
2018-10-11  8:55                   ` Michal Hocko
2018-10-11 17:38                     ` Alexander Duyck
2018-10-11 18:22                       ` Dan Williams
2018-10-17  7:52                       ` Michal Hocko
2018-10-17 15:02                         ` Alexander Duyck
2018-10-29 14:12                           ` Michal Hocko
2018-10-29 15:59                             ` Alexander Duyck
2018-10-29 16:35                               ` Michal Hocko
2018-10-29 17:01                                 ` Alexander Duyck
2018-10-29 17:24                                   ` Michal Hocko
2018-10-29 17:34                                     ` Dan Williams
2018-10-29 17:45                                       ` Michal Hocko
2018-10-29 17:42                                     ` Alexander Duyck
2018-10-29 18:18                                       ` Michal Hocko
2018-10-29 19:59                                         ` Alexander Duyck
2018-10-30  6:29                                           ` Michal Hocko
2018-10-30  6:55                                             ` Dan Williams
2018-10-30  8:17                                               ` Michal Hocko
2018-10-30 15:57                                                 ` Dan Williams
2018-10-30  8:05                                           ` Oscar Salvador
2018-10-29 15:49                           ` Dan Williams
2018-10-29 15:56                             ` Michal Hocko
2018-10-10 18:18               ` Dan Williams
2018-10-11  8:39                 ` Yi Zhang
2018-10-11 15:38                   ` Alexander Duyck

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