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b=fSAB7NW20JCAiLixJGlTa7CjLWqMJDWZgD12mgzeun5xFvFsWyDR4GqfEVYbvrC/I iEaKWjR+VSugmhXEWPc4hQEATJoflD5rXgGsRgBfnvOYuvFNYDG5EyqlDLXsvSBCk0 1Ga79NvuSti/pKjA4DyNTyQyjZ2QNjOy/ktmfcPbBLoDlzFNO5+ERlL9SY/xhzZ3sY ldcWLD89xFA/9xXjarUgBK6hR2JhiUjG//L5CECQs6tF5Rh2g3grJy07Xuq7bp1nKn lEPDLUOHiuBjmlAGPzS/NbnBZFx/dl/WLpcBbhItHFJtPVuIRyXs+X9OCeX6BHtGe6 EW0/Ue3RbSTRw== Message-ID: <805cfff2-1856-4b48-9afc-174a56405283@kernel.org> Date: Mon, 19 Oct 2026 18:41:50 +0200 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] cgroup/cpuset: use mems_allowed_seq when checking a policy nodemask Content-Language: en-US To: "Gregory Price (Meta)" , linux-mm@kvack.org, cgroups@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, longman@redhat.com, ridong.chen@linux.dev, tj@kernel.org, hannes@cmpxchg.org, mkoutny@suse.com, akpm@linux-foundation.org, mgorman@suse.de References: <20261019162138.3271947-1-gourry@gourry.net> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 340F6100007 X-Stat-Signature: 4ptkx3sd84461zh7qsssgsoqibe9fm7a X-Spam-Status: No, score=-17.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI, RCVD_IN_DNSWL_BLOCKED,RCVD_IN_ZEN_BLOCKED_OPENDNS,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspam-User: X-HE-Tag: 1792428115-289124 X-HE-Meta: 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 /7zVAIuL 3CdtttubvmTkBCWsKProM2CWv+3IcGneyUoZYXT8FOhjS0m3a56tWb0wd5e3oBRmnwSCA3OWg1gik4g9yzMh1ceExag6HGY1TTiiO5C0YS3J27Nrht/QOJiqf//smNbAvdpSdtaYZzDopLCVOG5gZq+ljowqhH6+cdIKFK4WO1MfxhT5qdHgQanFCi0FCB9rV87MWfy7KRhsRuWSQkXCLgJlJafzKdSnlj5O0cn8s6dyLJzo9+I/0AQNz+EmGs6wCzEfVKuCDj/wb9WdIqSZ3XEu2heBfbC/YcVpl1U1jLpNgDx071RhD9pDDsbJBjuXfevlOoHjqpA8bDPWNJDT28uNQuVKCtZS8e/RlitsQ/e4jDISajY6J/EAYWBeYxi8R1zh7wB5BmOvgsRdOWEmuZinKIo/7uEbczBI+ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 10/19/26 18:21, Gregory Price (Meta) wrote: > A cpuset update changes the task's mems_allowed and rebinds its > mempolicy in separate steps under mems_allowed_seq, but the check > reads both without it. This can cause a spurious numa placement > miss under some cpuset/mempolicy combinations. > > A reader that loads one mask before the update and the other after > it can see a spurious empty intersection and drop the nodemask. > > For example, a task with cpuset.mems={0} and MPOL_BIND={0}. If the > cpuset moves to mems={1,2}, the policy is rebound to {1}. > > The allocator can race and compare the old mems_allowed {0} against > the new policy {1} - which will result in a dropped nodemask and a > misplaced allocation on node 2. And since this can have only performance implications and not cause a functional issue, we don't need Cc: stable. (unlike violating a __GFP_THISNODE allocation which could be quite bad and has happened in the past) > Simply retry the intersection until mems_allowed_seq is stable. > > This covers the task policy, which is rebound under mems_allowed_seq. > VMA policies are rebound afterwards by mpol_rebind_mm(), outside > mems_allowed_seq, and are not covered by this change. > > Fixes: cc9a6c877661 ("cpuset: mm: reduce large amounts of memory barrier related damage v3") > Reviewed-by: Vlastimil Babka (SUSE) > Reviewed-by: Waiman Long > Signed-off-by: Gregory Price (Meta) > --- > v2: there is no real update here, i'm breaking it off of the page_alloc > series so the page_alloc fixes backport collectively cleaner. > > kernel/cgroup/cpuset.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > index 3f52717c19654..8ee239d5c6b48 100644 > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -4354,10 +4354,21 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk) > * @nodemask: the nodemask to be checked > * > * Are any of the nodes in the nodemask allowed in current->mems_allowed? > + * > + * A cpuset update rebinds the task's mempolicy and changes mems_allowed in > + * separate steps, so retry if an update raced with the check. > */ > int cpuset_nodemask_valid_mems_allowed(const nodemask_t *nodemask) > { > - return nodes_intersects(*nodemask, current->mems_allowed); > + unsigned int seq; > + int ret; > + > + do { > + seq = read_mems_allowed_begin(); > + ret = nodes_intersects(*nodemask, current->mems_allowed); > + } while (read_mems_allowed_retry(seq)); > + > + return ret; > } > > /*