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Mon, 20 Jul 2026 02:12:02 -0700 (PDT) Date: Mon, 20 Jul 2026 17:11:58 +0800 From: Kairui Song To: Kemeng Shi Cc: chrisl@kernel.org, kasong@tencent.com, nphamcs@gmail.com, baoquan.he@linux.dev, baohua@kernel.org, youngjun.park@lge.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/4] mm, swap: Fix potential NULL dereference when trying a sleep table allocation Message-ID: References: <20260720071342.50742-1-shikemeng@huaweicloud.com> <20260720071342.50742-2-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: <20260720071342.50742-2-shikemeng@huaweicloud.com> On Mon, Jul 20, 2026 at 03:13:39PM +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 updating allocated tables in atomic context. Thanks, that's a really tricky one, good catch. > > Fixes: 2fe7a6f5024b8 ("mm/memcg, swap: store cgroup id in cluster table directly") This commit doesn't exist, you mean b197d41462c2 right? > Signed-off-by: Kemeng Shi > --- > mm/swapfile.c | 21 ++++++++++++++++++++- > 1 file changed, 20 insertions(+), 1 deletion(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 615d90867111..d29062d9c3cd 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -490,6 +490,20 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp) > return 0; > } > > +static void swap_cluster_copy_table(struct swap_cluster_info *d_ci, > + struct swap_cluster_info *s_ci) Nit: The name is a bit misleading, it not copying the table content, just filled the d_ci with s_ci. And do we really need a helper for this? > +{ > + rcu_assign_pointer(d_ci->table, rcu_access_pointer(s_ci->table)); > + > +#ifdef CONFIG_MEMCG > + d_ci->memcg_table = s_ci->memcg_table; > +#endif > + > +#if !SWAP_TABLE_HAS_ZEROFLAG > + d_ci->zero_bitmap = s_ci->zero_bitmap; > +#endif > +} > + > /* > * Sanity check to ensure nothing leaked, and the specified range is empty. > * One special case is that bad slots can't be freed, so check the number of > @@ -527,6 +541,7 @@ static struct swap_cluster_info * > swap_cluster_populate(struct swap_info_struct *si, > struct swap_cluster_info *ci) > { > + struct swap_cluster_info tmp_ci; > int ret; > > /* > @@ -552,7 +567,8 @@ swap_cluster_populate(struct swap_info_struct *si, > spin_unlock(&si->global_cluster_lock); > local_unlock(&percpu_swap_cluster.lock); > > - ret = swap_cluster_alloc_table(ci, __GFP_HIGH | __GFP_NOMEMALLOC | > + tmp_ci = *ci; > + ret = swap_cluster_alloc_table(&tmp_ci, __GFP_HIGH | __GFP_NOMEMALLOC | > GFP_KERNEL); This allocation could be largely simplified with mempool, the reason I didn't use that is due to lack of RCU support, which can be done later to clean this up. Please fix the Fixes tags, other parts are mostly fine, thanks agian!