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Wed, 25 Sep 2019 19:48:56 +0000 (UTC) Received: from [10.36.116.51] (ovpn-116-51.ams2.redhat.com [10.36.116.51]) by smtp.corp.redhat.com (Postfix) with ESMTP id 902A01001B12; Wed, 25 Sep 2019 19:48:54 +0000 (UTC) Subject: Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock To: Qian Cai , Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Oscar Salvador , Pavel Tatashin , Dan Williams , Thomas Gleixner References: <20190924143615.19628-1-david@redhat.com> <1569337401.5576.217.camel@lca.pw> <20190924151147.GB23050@dhcp22.suse.cz> <1569351244.5576.219.camel@lca.pw> <2f8c8099-8de0-eccc-2056-a79d2f97fbf7@redhat.com> <1569427262.5576.225.camel@lca.pw> <20190925174809.GM23050@dhcp22.suse.cz> <1569435659.5576.227.camel@lca.pw> From: David Hildenbrand 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: <92bce3d4-0a3e-e157-529d-35aafbc30f3b@redhat.com> Date: Wed, 25 Sep 2019 21:48:53 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <1569435659.5576.227.camel@lca.pw> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Wed, 25 Sep 2019 19:48:56 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 25.09.19 20:20, Qian Cai wrote: > On Wed, 2019-09-25 at 19:48 +0200, Michal Hocko wrote: >> On Wed 25-09-19 12:01:02, Qian Cai wrote: >>> On Wed, 2019-09-25 at 09:02 +0200, David Hildenbrand wrote: >>>> On 24.09.19 20:54, Qian Cai wrote: >>>>> On Tue, 2019-09-24 at 17:11 +0200, Michal Hocko wrote: >>>>>> On Tue 24-09-19 11:03:21, Qian Cai wrote: >>>>>> [...] >>>>>>> While at it, it might be a good time to rethink the whole locking= over there, as >>>>>>> it right now read files under /sys/kernel/slab/ could trigger a p= ossible >>>>>>> deadlock anyway. >>>>>>> >>>>>> >>>>>> [...] >>>>>>> [=C2=A0=C2=A0442.452090][ T5224] -> #0 (mem_hotplug_lock.rw_sem){= ++++}: >>>>>>> [=C2=A0=C2=A0442.459748][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0validate_chain+0xd10/0x2bcc >>>>>>> [=C2=A0=C2=A0442.464883][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0__lock_acquire+0x7f4/0xb8c >>>>>>> [=C2=A0=C2=A0442.469930][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0lock_acquire+0x31c/0x360 >>>>>>> [=C2=A0=C2=A0442.474803][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0get_online_mems+0x54/0x150 >>>>>>> [=C2=A0=C2=A0442.479850][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0show_slab_objects+0x94/0x3a8 >>>>>>> [=C2=A0=C2=A0442.485072][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0total_objects_show+0x28/0x34 >>>>>>> [=C2=A0=C2=A0442.490292][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0slab_attr_show+0x38/0x54 >>>>>>> [=C2=A0=C2=A0442.495166][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0sysfs_kf_seq_show+0x198/0x2d4 >>>>>>> [=C2=A0=C2=A0442.500473][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0kernfs_seq_show+0xa4/0xcc >>>>>>> [=C2=A0=C2=A0442.505433][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0seq_read+0x30c/0x8a8 >>>>>>> [=C2=A0=C2=A0442.509958][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0kernfs_fop_read+0xa8/0x314 >>>>>>> [=C2=A0=C2=A0442.515007][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0__vfs_read+0x88/0x20c >>>>>>> [=C2=A0=C2=A0442.519620][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0vfs_read+0xd8/0x10c >>>>>>> [=C2=A0=C2=A0442.524060][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0ksys_read+0xb0/0x120 >>>>>>> [=C2=A0=C2=A0442.528586][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0__arm64_sys_read+0x54/0x88 >>>>>>> [=C2=A0=C2=A0442.533634][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0el0_svc_handler+0x170/0x240 >>>>>>> [=C2=A0=C2=A0442.538768][ T5224]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0el0_svc+0x8/0xc >>>>>> >>>>>> I believe the lock is not really needed here. We do not deallocate= d >>>>>> pgdat of a hotremoved node nor destroy the slab state because an >>>>>> existing slabs would prevent hotremove to continue in the first pl= ace. >>>>>> >>>>>> There are likely details to be checked of course but the lock just= seems >>>>>> bogus. >>>>> >>>>> Check 03afc0e25f7f ("slab: get_online_mems for >>>>> kmem_cache_{create,destroy,shrink}"). It actually talk about the ra= ces during >>>>> memory as well cpu hotplug, so it might even that cpu_hotplug_lock = removal is >>>>> problematic? >>>>> >>>> >>>> Which removal are you referring to? get_online_mems() does not mess = with >>>> the cpu hotplug lock (and therefore this patch). >>> >>> The one in your patch. I suspect there might be races among the whole= NUMA node >>> hotplug, kmem_cache_create, and show_slab_objects(). See bfc8c90139eb= ("mem- >>> hotplug: implement get/put_online_mems") >>> >>> "kmem_cache_{create,destroy,shrink} need to get a stable value of cpu= /node >>> online mask, because they init/destroy/access per-cpu/node kmem_cache= parts, >>> which can be allocated or destroyed on cpu/mem hotplug." >> >> I still have to grasp that code but if the slub allocator really needs >> a stable cpu mask then it should be using the explicit cpu hotplug >> locking rather than rely on side effect of memory hotplug locking. >> >>> Both online_pages() and show_slab_objects() need to get a stable valu= e of >>> cpu/node online mask. >> >> Could tou be more specific why online_pages need a stable cpu online >> mask? I do not think that show_slab_objects is a real problem because = a >> potential race shouldn't be critical. >=20 > build_all_zonelists() > __build_all_zonelists() > for_each_online_cpu(cpu) >=20 Two things: a) We currently always hold the device hotplug lock when onlining memory and when onlining cpus (for CPUs at least via user space - we would have to double check other call paths). So theoretically, that should guard us from something like that already. b) commit 11cd8638c37f6c400cc472cc52b6eccb505aba6e Author: Michal Hocko Date: Wed Sep 6 16:20:34 2017 -0700 mm, page_alloc: remove stop_machine from build_all_zonelists Tells me: "Updates of the zonelists happen very seldom, basically only when a zone becomes populated during memory online or when it loses all the memory during offline. A racing iteration over zonelists could either miss a zone or try to work on one zone twice. Both of these are something we can live with occasionally because there will always be at least one zone visible so we are not likely to fail allocation too easily for example." Sounds like if there would be a race, we could live with it if I am not getting that totally wrong. --=20 Thanks, David / dhildenb