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b=E9uwC3noUffTCjqknAzvm/sSeJXawABhkRsSILrPn17ih/UNq+c9+BkJD+Vkcn/hn N5bBuOUgg+FUmzKQnsE/oVMZYLCCx9hrW6Yl7KgZjHKHkRGIUqwXwExRpSdnFr1lmH CV9Fejyqa+F596cG/Rp0MUnKxO/YvIG6pWVKqXGu9IJR/Mbpn/1qF1W+iXNoEzYWu/ Fy51qJhpbToiLfgSvGECd+frmrnp9cXzd1yYGqL/Fg3Qlml0dOLSPHg7+a3XSASnH/ zvW0vmBxXA1GR+of9X+gR8TMmn6ZJGA8/W6aJIyJDqnhZRiL2xSqipW4HAshyA+atu Ief3OVsouqFIQ== Message-ID: <8d3b4561-92cd-4ebc-8462-5fb0fd659e8a@kernel.org> Date: Mon, 15 Jun 2026 10:08:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed To: Farhad Alemi , Andrew Morton , Waiman Long Cc: Farhad Alemi , Gregory Price , Yury Norov , Joshua Hahn , Zi Yan , Matthew Brost , Rakie Kim , Byungchul Park , Ying Huang , Alistair Popple , Rasmus Villemoes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, stable@vger.kernel.org References: From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 2E9D3140004 X-Stat-Signature: 61n4fhzq1xbpgs4eucgus6md7imhqkfa X-HE-Tag: 1781510938-182693 X-HE-Meta: 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 vGPIRiKn 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/14/26 15:25, Farhad Alemi wrote: Hi, thanks for your patch! For the future, please don't submit new revisions as reply to previous submissions. > Creating a child cpuset where cpuset.mems is never set leads to a div/0 > when a VMA mempolicy with MPOL_F_RELATIVE_NODES rebinds in response to a > CPU hotplug event. > > Reproduction steps: > 1) Create a cgroup w/ cpuset controls (do not set cpuset.mems) > 2) Move the task into the child cpuset > 3) Create a VMA mempolicy for that task with MPOL_F_RELATIVE_NODES > 4) unplug and hotplug a cpu > echo 0 > /sys/devices/system/cpu/cpu1/online > echo 1 > /sys/devices/system/cpu/cpu1/online > 5) mempolicy rebind does a div/0 in mpol_relative_nodemask on the > call to __nodes_fold() > > The cpuset code passes (cs->mems_allowed) which is not guaranteed to have > nodes to the rebind routine. Use cs->effective_mems instead, which is > guaranteed to have a non-empty nodemask. Probably worth mentioning here that this makes the linked reproducer happy. > > Link: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/ This should be a Closes: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/ > Link: https://lore.kernel.org/all/CA+0ovCiEz6SP_sn3kN4Tb+_oC=eHMXy_Ffj=usV3wREdQrUtww@mail.gmail.com/ > Fixes: ae1c802382f7 ("cpuset: apply cs->effective_{cpus,mems}") > Suggested-by: Gregory Price > Suggested-by: Waiman Long > Signed-off-by: Farhad Alemi > Cc: stable@vger.kernel.org > --- > v2: rebind to cs->effective_mems instead of newmems (Waiman Long); > condense the changelog. > > kernel/cgroup/cpuset.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -2649,7 +2649,7 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs) > > migrate = is_memory_migrate(cs); > > - mpol_rebind_mm(mm, &cs->mems_allowed); > + mpol_rebind_mm(mm, &cs->effective_mems); God this is confusing. So, we obtain newmems from guarantee_online_mems(), which guarantees that newmems is non-empty. In cpuset_change_task_nodemask(), we set tsk->mems_allowed to newmems, and call mpol_rebind_task(tsk, newmems). So at least tsk->mems_allowed should be non-empty. Then we call mpol_rebind_mm(mm, &cs->mems_allowed); Naturally I wonder: Why are we not using "task->mems_allowed" (maybe cs vs. tsk was the original bug?), which is effectively just newmems? guarantee_online_mems() computes newmems as "cs->effective_mems & node_states[N_MEMORY]", but walks up to the parent if it would be empty. -- Cheers, David