Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap
To:     Oscar Salvador <osalvador@techadventures.net>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
        pavel.tatashin@microsoft.com, dave.jiang@intel.com,
        dave.hansen@intel.com, jglisse@redhat.com, rppt@linux.vnet.ibm.com,
        dan.j.williams@intel.com, logang@deltatee.com, mingo@kernel.org,
        kirill.shutemov@linux.intel.com
References: <20180925200551.3576.18755.stgit@localhost.localdomain>
 <20180925202053.3576.66039.stgit@localhost.localdomain>
 <20180926075540.GD6278@dhcp22.suse.cz>
 <6f87a5d7-05e2-00f4-8568-bb3521869cea@linux.intel.com>
 <20180927110926.GE6278@dhcp22.suse.cz>
 <20180927122537.GA20378@techadventures.net>
 <20180927131329.GI6278@dhcp22.suse.cz>
 <20180927145035.GA21373@techadventures.net>
From:   David Hildenbrand <david@redhat.com>
Openpgp: preference=signencrypt
Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI
 BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG
 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj
 xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3
 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob
 s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM
 m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez
 MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK
 z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh
 dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E
 UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq
 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga
 uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid
 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X
 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+
 xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC
 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb
 hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC
 u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS
 gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg
 rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl
 BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl
 KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ
 NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo
 YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5
 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy
 qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl
 C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV
 W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV
 TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx
 +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl
 SE+xAvmumFBY
Organization: Red Hat GmbH
Message-ID: <27f1265b-a8b4-b203-077b-468d1e823cc8@redhat.com>
Date:   Thu, 27 Sep 2018 17:41:19 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.0
MIME-Version: 1.0
In-Reply-To: <20180927145035.GA21373@techadventures.net>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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