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([2405:201:682f:389d:7751:dfe4:62a3:f9d2]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2a3e3cc8ddfsm170993495ad.74.2026.01.12.02.36.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Jan 2026 02:36:18 -0800 (PST) From: Deepanshu Kartikey To: akpm@linux-foundation.org, urezki@gmail.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Deepanshu Kartikey , syzbot+d8d4c31d40f868eaea30@syzkaller.appspotmail.com Subject: [PATCH v2] mm/vmalloc: prevent RCU stalls in kasan_release_vmalloc_node Date: Mon, 12 Jan 2026 16:06:12 +0530 Message-ID: <20260112103612.627247-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" When CONFIG_PAGE_OWNER is enabled, freeing KASAN shadow pages during vmalloc cleanup triggers expensive stack unwinding that acquires RCU read locks. Processing a large purge_list without rescheduling can cause the task to hold CPU for extended periods (10+ seconds), leading to RCU stalls and potential OOM conditions. The issue manifests in purge_vmap_node() -> kasan_release_vmalloc_node() where iterating through hundreds or thousands of vmap_area entries and freeing their associated shadow pages causes: rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: rcu: Tasks blocked on level-0 rcu_node (CPUs 0-1): P6229/1:b..l ... task:kworker/0:17 state:R running task stack:28840 pid:6229 ... kasan_release_vmalloc_node+0x1ba/0xad0 mm/vmalloc.c:2299 purge_vmap_node+0x1ba/0xad0 mm/vmalloc.c:2299 Each call to kasan_release_vmalloc() can free many pages, and with page_owner tracking, each free triggers save_stack() which performs stack unwinding under RCU read lock. Without yielding, this creates an unbounded RCU critical section. Add periodic cond_resched() calls within the loop to allow: - RCU grace periods to complete - Other tasks to run - Scheduler to preempt when needed The fix uses need_resched() for immediate response under load, with a batch count of 32 as a guaranteed upper bound to prevent worst-case stalls even under light load. Reported-by: syzbot+d8d4c31d40f868eaea30@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3Dd8d4c31d40f868eaea30 Link: https://lore.kernel.org/all/20260112084723.622910-1-kartikey406@gmail= .com/T/ [v1] Suggested-by: Uladzislau Rezki Signed-off-by: Deepanshu Kartikey Reviewed-by: Uladzislau Rezki (Sony) --- v2: Use a macro for batch size (suggested by Uladzislau Rezki) --- mm/vmalloc.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/mm/vmalloc.c b/mm/vmalloc.c index 41dd01e8430c..51e58701565d 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -2268,11 +2268,14 @@ decay_va_pool_node(struct vmap_node *vn, bool full_= decay) reclaim_list_global(&decay_list); } =20 +#define KASAN_RELEASE_BATCH_SIZE 32 + static void kasan_release_vmalloc_node(struct vmap_node *vn) { struct vmap_area *va; unsigned long start, end; + unsigned int batch_count =3D 0; =20 start =3D list_first_entry(&vn->purge_list, struct vmap_area, list)->va_s= tart; end =3D list_last_entry(&vn->purge_list, struct vmap_area, list)->va_end; @@ -2282,6 +2285,11 @@ kasan_release_vmalloc_node(struct vmap_node *vn) kasan_release_vmalloc(va->va_start, va->va_end, va->va_start, va->va_end, KASAN_VMALLOC_PAGE_RANGE); + + if (need_resched() || (++batch_count >=3D KASAN_RELEASE_BATCH_SIZE)) { + cond_resched(); + batch_count =3D 0; + } } =20 kasan_release_vmalloc(start, end, start, end, KASAN_VMALLOC_TLB_FLUSH); --=20 2.43.0