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Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1w38Ex-0000000CyQr-2Ztq; Thu, 19 Mar 2026 07:56:23 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id F141F301BD5; Thu, 19 Mar 2026 08:56:21 +0100 (CET) Date: Thu, 19 Mar 2026 08:56:21 +0100 From: Peter Zijlstra To: Nhat Pham Cc: kasong@tencent.com, Liam.Howlett@oracle.com, akpm@linux-foundation.org, apopple@nvidia.com, axelrasmussen@google.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, bhe@redhat.com, byungchul@sk.com, cgroups@vger.kernel.org, chengming.zhou@linux.dev, chrisl@kernel.org, corbet@lwn.net, david@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jannh@google.com, joshua.hahnjy@gmail.com, lance.yang@linux.dev, lenb@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-pm@vger.kernel.org, lorenzo.stoakes@oracle.com, matthew.brost@intel.com, mhocko@suse.com, muchun.song@linux.dev, npache@redhat.com, pavel@kernel.org, peterx@redhat.com, pfalcato@suse.de, rafael@kernel.org, rakie.kim@sk.com, roman.gushchin@linux.dev, rppt@kernel.org, ryan.roberts@arm.com, shakeel.butt@linux.dev, shikemeng@huaweicloud.com, surenb@google.com, tglx@kernel.org, vbabka@suse.cz, weixugc@google.com, ying.huang@linux.alibaba.com, yosry.ahmed@linux.dev, yuanchu@google.com, zhengqi.arch@bytedance.com, ziy@nvidia.com, kernel-team@meta.com, riel@surriel.com Subject: Re: [PATCH v4 09/21] mm: swap: allocate a virtual swap slot for each swapped out page Message-ID: <20260319075621.GR3738010@noisy.programming.kicks-ass.net> References: <20260318222953.441758-1-nphamcs@gmail.com> <20260318222953.441758-10-nphamcs@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260318222953.441758-10-nphamcs@gmail.com> X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 2BDD21C0003 X-Stat-Signature: fshwi9b36hhophq1i6eyaxagiwmgjjak X-HE-Tag: 1773907010-265413 X-HE-Meta: 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 bjmGwH7J X2ikWwLDKti6DnGzyPn6efZRh6QW+hC8FnBbOM/nY9uHG/PTx26RBdbnRx2aOpnBbDRkX1/zWqzTaXuOPw9HhmweCa/YPtxehLRm6nGff9i09ZIksNkRWytPWvf9kzyqCE6ryGM3VqjVu++bLQALLo+IGqpP2/59ca70NNWVnGcf+bQD9zgu+bVhhRXavawf6Wk8vDdfD6dEA38gjP7s7u2gqqMRsN2iBABgBxjPFMsjWm3AzOm0aUI2iwg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Mar 18, 2026 at 03:29:40PM -0700, Nhat Pham wrote: > diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h > index 62cd7b35a29c9..85cb45022e796 100644 > --- a/include/linux/cpuhotplug.h > +++ b/include/linux/cpuhotplug.h > @@ -86,6 +86,7 @@ enum cpuhp_state { > CPUHP_FS_BUFF_DEAD, > CPUHP_PRINTK_DEAD, > CPUHP_MM_MEMCQ_DEAD, > + CPUHP_MM_VSWAP_DEAD, > CPUHP_PERCPU_CNT_DEAD, > CPUHP_RADIX_DEAD, > CPUHP_PAGE_ALLOC, > +static int vswap_cpu_dead(unsigned int cpu) > +{ > + struct vswap_cluster *cluster; > + int order; > + > + rcu_read_lock(); nit: guard(rcu)(); > + for (order = 0; order < SWAP_NR_ORDERS; order++) { > + cluster = per_cpu(percpu_vswap_cluster.clusters[order], cpu); > + if (cluster) { > + per_cpu(percpu_vswap_cluster.clusters[order], cpu) = NULL; > + spin_lock(&cluster->lock); This breaks on PREEMPT_RT as this is ran with IRQs disabled. This must be a raw_spinlock_t. > + cluster->cached = false; > + if (refcount_dec_and_test(&cluster->refcnt)) > + vswap_cluster_free(cluster); And this... below. > + spin_unlock(&cluster->lock); > + } > + } > + rcu_read_unlock(); > + > + return 0; > +} > +static void vswap_cluster_free(struct vswap_cluster *cluster) > +{ > + VM_WARN_ON(cluster->count || cluster->cached); > + VM_WARN_ON(!spin_is_locked(&cluster->lock)); This is terrible, please use: lockdep_assert_held(&cluster->lock); > + xa_lock(&vswap_cluster_map); This is again broken, this cannot be from a DEAD callback with IRQs disabled. > + list_del_init(&cluster->list); > + __xa_erase(&vswap_cluster_map, cluster->id); Strictly speaking this can end up in xas_alloc(), which is again, not allowed in a DEAD callback. > + xa_unlock(&vswap_cluster_map); > + rcu_head_init(&cluster->rcu); > + kvfree_rcu(cluster, rcu); > +}