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Thu, 27 Aug 2026 02:10:58 -0700 (PDT) Date: Thu, 27 Aug 2026 17:10:54 +0800 From: Kairui Song To: Kemeng Shi Cc: akpm@linux-foundation.org, chrisl@kernel.org, kasong@tencent.com, nphamcs@gmail.com, baoquan.he@linux.dev, youngjun.park@lge.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/4] mm, swap: Fix potential NULL dereference when trying a sleep table allocation Message-ID: References: <20260820115505.20027-1-shikemeng@huaweicloud.com> <20260820115505.20027-2-shikemeng@huaweicloud.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260820115505.20027-2-shikemeng@huaweicloud.com> On Thu, Aug 20, 2026 at 07:55:02PM +0800, Kemeng Shi wrote: > The root cause of this issue is because multi-tables are updated in non > atomic context. To be more specific, the issue could be triggerred as > following: > > swap_alloc_fast swap_cluster_populate() > /* Try a sleep allocation */ > spin_unlock(&ci->lock); > swap_cluster_alloc_table() > rcu_assign_pointer(ci->table, table); > > ci = swap_cluster_lock(si, offset) > cluster_is_usable(ci, order） > if (!cluster_table_is_alloced(ci)) // ok > alloc_swap_scan_cluster() > cluster_scan_range() > __swap_table_get() > > /* free table when more table allocation fails */ > ci->memcg_table = kzalloc_obj(*ci->memcg_table, > gfp); > if (!ci->memcg_table) > swap_cluster_free_table() > rcu_assign_pointer(ci->table, NULL); > > table = rcu_dereference_check(ci->table, lockdep_is_held(&ci->lock)); > atomic_long_read(&table[off]); // NULL dereference > > Fix the issue by making tables visible at end of swap_cluster_populate(). I think you mean "end of swap_cluster_alloc_table". > > Fixes: 2fe7a6f5024b8 ("mm/memcg, swap: store cgroup id in cluster table directly") Hmm, wrong commit id again, should be Fixes: b197d41462c2 ("mm/memcg, swap: store cgroup id in cluster table directly") > Signed-off-by: Kemeng Shi > --- > mm/swapfile.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 615d90867111..4561c864f806 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -466,8 +466,6 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp) > if (!table) > return -ENOMEM; > > - rcu_assign_pointer(ci->table, table); > - > #ifdef CONFIG_MEMCG > if (!mem_cgroup_disabled()) { > VM_WARN_ON_ONCE(ci->memcg_table); > @@ -487,6 +485,12 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp) > return -ENOMEM; > } > #endif > + > + /* > + * Make tables visible to cluster_is_usable() after everything is > + * ready. > + */ > + rcu_assign_pointer(ci->table, table); Hmm, but for the error paths above, they will leak the new allocated table?