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t=1793350517; bh=2ME/YKcQAmqEKXR0yxRYRyIXblEy16rwwZK5YWNyjhA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=CXll4QP4+ROaktyPv7RAJz7f+WPuofJbzyw2m4AACejiGVIILiYZq+PxNbpYDu8vd 2Dm5iuJ/12bzLkrDS1RdE10rGH6i9HYqGkE0UXGfgzXOzXXE6naiSTFXFT7mi2yMsS a5WNcQ+GTLBcmmRjtarEdx1R8caixrVhQKolICv40IegTz6IRR+SoElmpwOTHyW/q6 7Df3TV3klPlzpPUtvCGhWSAq9qOAqVDLWQWzyNS7wv5Qwyy8WwoASqZBNjn4XEEStK CMKfPt8dwsnEchT7An+jO6D/nNxutbcvk8hJ6ADyAH3HF0ZZuRBHVTr7HFOcvGhjm2 gDwN1kw1sLmow== Message-ID: <1e277f4a-87b7-4f5e-8c87-1b2bf280e859@kernel.org> Date: Fri, 30 Oct 2026 09:54:59 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 38/46] mm/ksm: reset the unstable trees of all nodes To: xu.xin@linux.dev Cc: akpm@linux-foundation.org, aneesh.kumar@kernel.org, apopple@nvidia.com, axelrasmussen@google.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, baoquan.he@linux.dev, chengming.zhou@linux.dev, chrisl@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, harry@kernel.org, hughd@google.com, jack@suse.cz, jannh@google.com, jgg@ziepe.ca, jhubbard@nvidia.com, josef@toxicpanda.com, joshua.hahnjy@gmail.com, kas@kernel.org, kasong@tencent.com, kees@kernel.org, kexec@lists.infradead.org, lance.yang@linux.dev, liam@infradead.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, matthew.brost@intel.com, mhocko@kernel.org, mhocko@suse.com, minchan@kernel.org, muchun.song@linux.dev, nico.pache@linux.dev, nphamcs@gmail.com, npiggin@gmail.com, osalvador@suse.de, pasha.tatashin@soleen.com, peterx@redhat.com, peterz@infradead.org, pfalcato@suse.de, pratyush@kernel.org, qi.zheng@linux.dev, rakie.kim@sk.com, riel@surriel.com, rppt@kernel.org, ryan.roberts@arm.com, shakeel.butt@linux.dev, shikemeng@huaweicloud.com, stable@vger.kernel.org, surenb@google.com, usama.arif@linux.dev, vbabka@kernel.org, weixugc@google.com, will@kernel.org, willy@infradead.org, ying.huang@linux.alibaba.com, youngjun.park@lge.com, yuanchu@google.com, ziy@nvidia.com References: <20261027-fixes_core-v1-38-6f74bec9a5bb@kernel.org> <20261030032409.127393-1-xu.xin@linux.dev> 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-Stat-Signature: knxpu5i575syyyh334sqfs1bhidyboyd X-Spam-Status: No, score=-17.3 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,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 919E840002 X-Rspam-User: X-HE-Tag: 1793350518-658039 X-HE-Meta: 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 MexscRov T0hjPcC0ufJPwVkzeVRk9JHy5rm1UM8E1DhG2kfvOazDSRIbkgDflElSPZtKNPWPgkXoimYiiBsLEfX3b5AHTw/TxA2GXjMRzuklixpjJx19onYBbqSYlZTGXUozkfcYsj7DlifNqwLpjPv+kTHLItUHPKd7L0F01LLbzM/2Wfh2/UJTlJ3E3NZgu15Y/YyRZr18zg56v/t3AlfwJMwJ+4vtEqMOLypTyqZq2n8rKvzagejb8IcrnGG7KuBGCosa6/lWwwFdLGeaqzy0UvB9kPqtN5TlgIPTqN93OVuCOB35Jpiq5eEm23VoLBm705ZRVne4CqhAMIWt6aoOroByi/ws37pn8EOSnPqYmy3ELbnZ18dHIywmr8pGyxwQDAHtlOjsnHuN/VvJshrEm1Z74P9SUhucnXOBzwGpM 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 10/30/26 04:24, xu.xin@linux.dev wrote: >> >> Let's fix it by tracking in ksm_nr_node_ids the actual number of >> allocated tree roots. >> >> remove_all_stable_nodes() and ksm_check_stable_tree() will now scan >> more tree roots (skipping the empty ones), which is harmless. >> >> Fixes: ef53d16cded7 ("ksm: allocate roots when needed") >> Cc: stable@vger.kernel.org >> Reported-by: Josef Bacik >> Signed-off-by: David Hildenbrand (Arm) >> --- >> mm/ksm.c | 6 +++--- >> 1 file changed, 3 insertions(+), 3 deletions(-) >> >> diff --git a/mm/ksm.c b/mm/ksm.c >> index dbfb8f98b1892..9c51925825cbf 100644 >> --- a/mm/ksm.c >> +++ b/mm/ksm.c >> @@ -475,6 +475,7 @@ static void advisor_stop_scan(void) >> #ifdef CONFIG_NUMA >> /* Zeroed when merging across nodes is not allowed */ >> static unsigned int ksm_merge_across_nodes = 1; >> +/* Number of allocated stable and unstable tree roots. */ >> static int ksm_nr_node_ids = 1; >> #else >> #define ksm_merge_across_nodes 1U >> @@ -3636,14 +3637,13 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj, >> else { >> root_stable_tree = buf; >> root_unstable_tree = buf + nr_node_ids; >> + ksm_nr_node_ids = nr_node_ids; >> /* Stable tree is empty but not the unstable */ >> root_unstable_tree[0] = one_unstable_tree[0]; >> } >> } >> - if (!err) { >> + if (!err) >> ksm_merge_across_nodes = knob; >> - ksm_nr_node_ids = knob ? 1 : nr_node_ids; >> - } >> } >> mutex_unlock(&ksm_thread_mutex); >> >> >> -- >> 2.43.0 > Thanks for the review! > Looks correct as far as it goes, but I think it is not quite sufficient > on its own. I think this fixes the problem described. What you describe below is a mismatch between documentation and the implementation, but I don't think it is problematic as in "responsible for BUGS". See below. > > The documentation requires that merge_across_nodes can only be changed > when there are no KSM shared pages in the system, but > merge_across_nodes_store() does not enforce this at the sysfs level. > > Should we also reject writes while ksmd is merging? At minimum: > > + if (ksm_run & KSM_RUN_MERGE) > + return -EBUSY; > > Ideally, we should also check that ksm_pages_shared is zero, since > KSM_RUN_STOP (0) does not guarantee that all pages have been unmerged. We do that already in merge_across_nodes_store() I think: if (ksm_pages_shared || remove_all_stable_nodes()) err = -EBUSY; Which is really required when swapping trees. > > That would make the interface match the documented sequence and close > the remaining race window. Right, either we should adjust documentation or adjust the implementation. Not sure if restricting the interface is really required. Personally, I'd simply fixup the documentation (separately) unless there is a real problem with the implementation when allowing KSM_RUN_MERGE. (I think there is no problem as we are synchronized on the mutex). -- Cheers, David