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 v1 1/2] powerpc/pseries/hotplug-memory: stop checking
 is_mem_section_removable()
To:     piliu <piliu@redhat.com>, Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linuxppc-dev@lists.ozlabs.org,
        Nathan Fontenot <nfont@linux.vnet.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Wei Yang <richard.weiyang@gmail.com>
References: <20200407135416.24093-1-david@redhat.com>
 <20200407135416.24093-2-david@redhat.com>
 <20200408024630.GQ2402@MiWiFi-R3L-srv>
 <16187f69-0e5b-c9c2-a31b-8658425758aa@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata=
 mQINBFXLn5EBEAC+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
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABtCREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT6JAlgEEwEIAEICGwMFCQlmAYAGCwkIBwMCBhUI
 AgkKCwQWAgMBAh4BAheAFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl3pImkCGQEACgkQTd4Q
 9wD/g1o+VA//SFvIHUAvul05u6wKv/pIR6aICPdpF9EIgEU448g+7FfDgQwcEny1pbEzAmiw
 zAXIQ9H0NZh96lcq+yDLtONnXk/bEYWHHUA014A1wqcYNRY8RvY1+eVHb0uu0KYQoXkzvu+s
 Dncuguk470XPnscL27hs8PgOP6QjG4jt75K2LfZ0eAqTOUCZTJxA8A7E9+XTYuU0hs7QVrWJ
 jQdFxQbRMrYz7uP8KmTK9/Cnvqehgl4EzyRaZppshruKMeyheBgvgJd5On1wWq4ZUV5PFM4x
 II3QbD3EJfWbaJMR55jI9dMFa+vK7MFz3rhWOkEx/QR959lfdRSTXdxs8V3zDvChcmRVGN8U
 Vo93d1YNtWnA9w6oCW1dnDZ4kgQZZSBIjp6iHcA08apzh7DPi08jL7M9UQByeYGr8KuR4i6e
 RZI6xhlZerUScVzn35ONwOC91VdYiQgjemiVLq1WDDZ3B7DIzUZ4RQTOaIWdtXBWb8zWakt/
 ztGhsx0e39Gvt3391O1PgcA7ilhvqrBPemJrlb9xSPPRbaNAW39P8ws/UJnzSJqnHMVxbRZC
 Am4add/SM+OCP0w3xYss1jy9T+XdZa0lhUvJfLy7tNcjVG/sxkBXOaSC24MFPuwnoC9WvCVQ
 ZBxouph3kqc4Dt5X1EeXVLeba+466P1fe1rC8MbcwDkoUo65Ag0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAGJAiUEGAECAA8FAlXLn5ECGwwFCQlmAYAACgkQTd4Q
 9wD/g1qA6w/+M+ggFv+JdVsz5+ZIc6MSyGUozASX+bmIuPeIecc9UsFRatc91LuJCKMkD9Uv
 GOcWSeFpLrSGRQ1Z7EMzFVU//qVs6uzhsNk0RYMyS0B6oloW3FpyQ+zOVylFWQCzoyyf227y
 GW8HnXunJSC+4PtlL2AY4yZjAVAPLK2l6mhgClVXTQ/S7cBoTQKP+jvVJOoYkpnFxWE9pn4t
 H5QIFk7Ip8TKr5k3fXVWk4lnUi9MTF/5L/mWqdyIO1s7cjharQCstfWCzWrVeVctpVoDfJWp
 4LwTuQ5yEM2KcPeElLg5fR7WB2zH97oI6/Ko2DlovmfQqXh9xWozQt0iGy5tWzh6I0JrlcxJ
 ileZWLccC4XKD1037Hy2FLAjzfoWgwBLA6ULu0exOOdIa58H4PsXtkFPrUF980EEibUp0zFz
 GotRVekFAceUaRvAj7dh76cToeZkfsjAvBVb4COXuhgX6N4pofgNkW2AtgYu1nUsPAo+NftU
 CxrhjHtLn4QEBpkbErnXQyMjHpIatlYGutVMS91XTQXYydCh5crMPs7hYVsvnmGHIaB9ZMfB
 njnuI31KBiLUks+paRkHQlFcgS2N3gkRBzH7xSZ+t7Re3jvXdXEzKBbQ+dC3lpJB0wPnyMcX
 FOTT3aZT7IgePkt5iC/BKBk3hqKteTnJFeVIT7EC+a6YUFg=
Organization: Red Hat GmbH
Message-ID: <85637e60-4d11-2b69-f2a9-1505e0342ce3@redhat.com>
Date:   Thu, 9 Apr 2020 09:26:22 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.6.0
MIME-Version: 1.0
In-Reply-To: <16187f69-0e5b-c9c2-a31b-8658425758aa@redhat.com>
Content-Type: text/plain; charset=windows-1252
Content-Language: en-US
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 09.04.20 04:59, piliu wrote:
>=20
>=20
> On 04/08/2020 10:46 AM, Baoquan He wrote:
>> Add Pingfan to CC since he usually handles ppc related bugs for RHEL.
>>
>> On 04/07/20 at 03:54pm, David Hildenbrand wrote:
>>> In commit 53cdc1cb29e8 ("drivers/base/memory.c: indicate all memory
>>> blocks as removable"), the user space interface to compute whether a =
memory
>>> block can be offlined (exposed via
>>> /sys/devices/system/memory/memoryX/removable) has effectively been
>>> deprecated. We want to remove the leftovers of the kernel implementat=
ion.
>>
>> Pingfan, can you have a look at this change on PPC?  Please feel free =
to
>> give comments if any concern, or offer ack if it's OK to you.
>>
>>>
>>> When offlining a memory block (mm/memory_hotplug.c:__offline_pages())=
,
>>> we'll start by:
>>> 1. Testing if it contains any holes, and reject if so
>>> 2. Testing if pages belong to different zones, and reject if so
>>> 3. Isolating the page range, checking if it contains any unmovable pa=
ges
>>>
>>> Using is_mem_section_removable() before trying to offline is not only=
 racy,
>>> it can easily result in false positives/negatives. Let's stop manuall=
y
>>> checking is_mem_section_removable(), and let device_offline() handle =
it
>>> completely instead. We can remove the racy is_mem_section_removable()
>>> implementation next.
>>>
>>> We now take more locks (e.g., memory hotplug lock when offlining and =
the
>>> zone lock when isolating), but maybe we should optimize that
>>> implementation instead if this ever becomes a real problem (after all=
,
>>> memory unplug is already an expensive operation). We started using
>>> is_mem_section_removable() in commit 51925fb3c5c9 ("powerpc/pseries:
>>> Implement memory hotplug remove in the kernel"), with the initial
>>> hotremove support of lmbs.
>>>
>>> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
>>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>>> Cc: Paul Mackerras <paulus@samba.org>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Oscar Salvador <osalvador@suse.de>
>>> Cc: Baoquan He <bhe@redhat.com>
>>> Cc: Wei Yang <richard.weiyang@gmail.com>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>>  .../platforms/pseries/hotplug-memory.c        | 26 +++--------------=
--
>>>  1 file changed, 3 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/p=
owerpc/platforms/pseries/hotplug-memory.c
>>> index b2cde1732301..5ace2f9a277e 100644
>>> --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
>>> +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
>>> @@ -337,39 +337,19 @@ static int pseries_remove_mem_node(struct devic=
e_node *np)
>>> =20
>>>  static bool lmb_is_removable(struct drmem_lmb *lmb)
>>>  {
>>> -	int i, scns_per_block;
>>> -	bool rc =3D true;
>>> -	unsigned long pfn, block_sz;
>>> -	u64 phys_addr;
>>> -
>>>  	if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
>>>  		return false;
>>> =20
>>> -	block_sz =3D memory_block_size_bytes();
>>> -	scns_per_block =3D block_sz / MIN_MEMORY_BLOCK_SIZE;
>>> -	phys_addr =3D lmb->base_addr;
>>> -
>>>  #ifdef CONFIG_FA_DUMP
>>>  	/*
>>>  	 * Don't hot-remove memory that falls in fadump boot memory area
>>>  	 * and memory that is reserved for capturing old kernel memory.
>>>  	 */
>>> -	if (is_fadump_memory_area(phys_addr, block_sz))
>>> +	if (is_fadump_memory_area(lmb->base_addr, memory_block_size_bytes()=
))
>>>  		return false;
>>>  #endif
>>> -
>>> -	for (i =3D 0; i < scns_per_block; i++) {
>>> -		pfn =3D PFN_DOWN(phys_addr);
>>> -		if (!pfn_in_present_section(pfn)) {
>>> -			phys_addr +=3D MIN_MEMORY_BLOCK_SIZE;
>>> -			continue;
>>> -		}
>>> -
>>> -		rc =3D rc && is_mem_section_removable(pfn, PAGES_PER_SECTION);
>>> -		phys_addr +=3D MIN_MEMORY_BLOCK_SIZE;
>>> -	}
>>> -
>>> -	return rc;
>>> +	/* device_offline() will determine if we can actually remove this l=
mb */
>>> +	return true;
> So I think here swaps the check and do sequence. At least it breaks
> dlpar_memory_remove_by_count(). It is doable to remove
> is_mem_section_removable(), but here should be more effort to re-arrang=
e
> the code.
>=20

Thanks Pingfan,

1. "swaps the check and do sequence":

Partially. Any caller of dlpar_remove_lmb() already has to deal with
false positives. device_offline() can easily fail after
dlpar_remove_lmb() =3D=3D true. It's inherently racy.

2. "breaks dlpar_memory_remove_by_count()"

Can you elaborate why it "breaks" it? It will simply try to
offline+remove lmbs, detect that it wasn't able to offline+remove as
much as it wanted (which could happen before as well easily), and re-add
the already offlined+removed ones.

3. "more effort to re-arrange the code"

What would be your suggestion?

We would rip out that racy check if we can remove as much memory as
requested in dlpar_memory_remove_by_count() and simply always try to
remove + recover.


--=20
Thanks,

David / dhildenb