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b=oBD2HdoalYSok5N4pY6kBCNdvjRU8dXJkrEkHVtHj0jBamypQY51munrUhhHGFuDD RA88QQCAyMV1zKjY47JZ31ieLBmm9a7WTQbMzjlW7CLaPP+M9HkY+fGpZiZW9awpu+ Mdlu/aJQ+QtjO9Rok7qlmqOw9rAqo3XO0T7R05P3R5hBdXjW7JWA0LPBT46HUn8+Px 61sqh4puubjmd0A/gA2JB2/lvvHNIWdHhUBZOn56IfVKPdqr0YMvtPcSVOlKYum7+L 8L2juZWZCPKId1o/AXgpM7nNZ8sFW2BeVGkpS+gALclZ7tmOkDPHn8HEcs5cdqK8dg bCC6fvyiQ8KAg== Message-ID: <87e0523c-1fc6-42aa-8159-150fd94d5b62@kernel.org> Date: Sun, 18 Jan 2026 19:45:11 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/mempolicy: fix mpol_rebind_nodemask() for MPOL_F_NUMA_BALANCING To: Jinjiang Tu , Andrew Morton Cc: akpm@linuxfoundation.org, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, mgorman@suse.de, linux-mm@kvack.org, wangkefeng.wang@huawei.com References: <20251223110523.1161421-1-tujinjiang@huawei.com> <04b92008-f843-4879-b4a3-608cc5e1de4c@kernel.org> <20260115101252.2e0cbe0559e62b988e5f7151@linux-foundation.org> <1ad31dbd-6743-473d-9f66-a603b91d1e54@huawei.com> <70d46998-a6c6-4c18-b8d7-f813582d3143@kernel.org> <7471b637-537c-40db-ade0-ad373d7085f7@huawei.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 49wxdjmanixq1f4bq6mns8zjkdg6zba8 X-Rspamd-Queue-Id: 6B42518000B X-Rspamd-Server: rspam04 X-HE-Tag: 1768761918-199300 X-HE-Meta: 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 FCCiQhua pZg12DO2vt7v5ZFXQQch0ZRBKuxjUukHmMXprfgJE99mZNvdSCM4gizU0QWqZPjmYn72JarGcfNpV/t0JpOk1djUFcF+rsxLRLjMaqs0+9756eNJNf20R/Ncct2Sm6DPE/xD1PFlgm4aeiBZQ+JM9RzQbXPabd35/bZBfZR3K/1AGL27AteclEfltnRHZRYFGX4qSZUCTKyAcyTCAUlBcaAtU1MMC2YdJAFqX85pAoWb5jbBIg4XLWN0NJ6wnUaU+HsT+SHlQLfczPSnwf7ECTvMWlWQYMK0zvtsstmt1QWAmHK3o5V/bqdCCc3fBjdamYWoPgumeexIZBR1A1jjK6auLF4N8/ej8+PmkuWc695I2nx7CHqu6vxggvt0j+geAR1dmqpNdIrHqImjdx66LEp40ujfDMLIIlXzbK9IdiMUEVzQ= 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: List-Subscribe: List-Unsubscribe: On 1/17/26 02:00, Jinjiang Tu wrote: > > 在 2026/1/16 18:58, David Hildenbrand (Red Hat) 写道: >> On 1/16/26 07:43, Jinjiang Tu wrote: >>> >>> 在 2026/1/16 2:12, Andrew Morton 写道: >>>> On Thu, 15 Jan 2026 18:10:51 +0100 "David Hildenbrand (Red Hat)" >>>> wrote: >>>> >>>>> On 12/23/25 12:05, Jinjiang Tu wrote: >>>>>> commit bda420b98505 ("numa balancing: migrate on fault among multiple >>>>>> bound nodes") adds new flag MPOL_F_NUMA_BALANCING to enable NUMA >>>>>> balancing >>>>>> for MPOL_BIND memory policy. >>>>>> >>>>>> When the cpuset of tasks changes, the mempolicy of the task is >>>>>> rebound by >>>>>> mpol_rebind_nodemask(). When MPOL_F_STATIC_NODES and >>>>>> MPOL_F_RELATIVE_NODES >>>>>> are both not set, the behaviour of rebinding should be same whenever >>>>>> MPOL_F_NUMA_BALANCING is set or not. So, when an application calls >>>>>> set_mempolicy() with MPOL_F_NUMA_BALANCING set but both >>>>>> MPOL_F_STATIC_NODES >>>>>> and MPOL_F_RELATIVE_NODES cleared, mempolicy.w.cpuset_mems_allowed >>>>>> should >>>>>> be set to cpuset_current_mems_allowed nodemask. However, in current >>>>>> implementation, mpol_store_user_nodemask() wrongly returns true, >>>>>> causing >>>>>> mempolicy->w.user_nodemask to be incorrectly set to the >>>>>> user-specified >>>>>> nodemask. Later, when the cpuset of the application changes, >>>>>> mpol_rebind_nodemask() ends up rebinding based on the user-specified >>>>>> nodemask rather than the cpuset_mems_allowed nodemask as intended. >>>>>> >>>>>> To fix this, only set mempolicy->w.user_nodemask to the >>>>>> user-specified >>>>>> nodemask if MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES is present. >>>>>> >>>>> ... >>>>> >>>>> I glimpsed over it and I think this is the right fix, thanks! >>>>> >>>>> Acked-by: David Hildenbrand (Red Hat) >>>> Cool.  I decided this was "not for backporting", but the description of >>>> the userspace-visible runtime effects isn't very clear. Jinjiang, can >>>> you please advise? >>> >>> I agree don't backport this patch. Users can only see tasks binding to >>> wrong NUMA after it's cpuset changes. >>> >>> Assuming there are 4 NUMA. task is binding to NUMA1 and it is in root >>> cpuset. >>> Move the task to a cpuset whose cpuset.mems.effective is 0-1. The >>> task should >>> still be binded to NUMA1, but is binded to NUMA0 wrongly. >> >> Do you think it's easy to write a reproducer to be run in a simple >> QEMU VM with 4 nodes? > > I can reproduce with the following steps: > > 1. echo '+cpuset' > /sys/fs/cgroup/cgroup.subtree_control > 2. mkdir /sys/fs/cgroup/test > 3. ./reproducer & > 4. cat /proc/$pid/numa_maps, the task is bound to NUMA 1 > 5. echo $pid > /sys/fs/cgroup/test/cgroup.procs > 6. cat /proc/$pid/numa_maps, the task is bound to NUMA 0 now. > > The reproducer code: > > int main() > { > struct bitmask *bmp; > int ret; > > bmp = numa_parse_nodestring("1"); > ret = set_mempolicy(MPOL_BIND | MPOL_F_NUMA_BALANCING, bmp->maskp, bmp->size + 1); > if (ret < 0) { > perror("Failed to call set_mempolicy"); > exit(-1); > } > > while (1); > return 0; > } > > If I call set_mempolicy() without MPOL_F_NUMA_BALANCING. After step 5, the task is still bound to NUMA 1. > Great, can you incorporate that into an updated patch description? And it might make sense to point at commit bda420b98505 ("numa balancing: migrate on fault among multiple bound nodes") where we document " we add MPOL_F_NUMA_BALANCING mode flag to set_mempolicy() when mode is MPOL_BIND. With the flag specified, NUMA balancing will be enabled within the thread to optimize the page placement within the constrains of the specified memory binding policy. " The "within the constrains" is the crucial bit here. -- Cheers David