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Date:   Mon, 28 Dec 2020 17:00:53 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Christian Brauner <christian.brauner@...ntu.com>, hch@...radead.org
Cc:     willy@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-aio@...ck.org,
        io-uring@...r.kernel.org
Subject: Re: Bug in __mmdrop() triggered by io-uring on v5.11-rc1

On 12/28/20 9:54 AM, Christian Brauner wrote:
> Hey everyone,
> 
> The following oops can be triggered on a pristine v5.11-rc1 which I discovered
> while rebasing my idmapped mount patchset onto v5.11-rc1:
> 
> [  577.716339][ T7216] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)/LXD, BIOS 0.0.0 02/06/2015
> [  577.718584][ T7216] Call Trace:
> [  577.719357][ T7216]  dump_stack+0x10b/0x167
> [  577.720505][ T7216]  panic+0x347/0x783
> [  577.721588][ T7216]  ? print_oops_end_marker.cold+0x15/0x15
> [  577.723502][ T7216]  ? __warn.cold+0x5/0x2f
> [  577.725079][ T7216]  ? __mmdrop+0x30c/0x400
> [  577.736066][ T7216]  __warn.cold+0x20/0x2f
> [  577.745503][ T7216]  ? __mmdrop+0x30c/0x400
> [  577.755101][ T7216]  report_bug+0x277/0x300
> 
> f2-vm login: [  577.764873][ T7216]  handle_bug+0x3c/0x60
> [  577.773982][ T7216]  exc_invalid_op+0x18/0x50
> [  577.786341][ T7216]  asm_exc_invalid_op+0x12/0x20
> [  577.795500][ T7216] RIP: 0010:__mmdrop+0x30c/0x400
> [  577.804426][ T7216] Code: 00 00 4c 89 ef e8 64 61 8c 02 eb 82 e8 dd 48 32 00 4c 89 e7 e8 35 97 2e 00 e9 70 ff ff ff e8 cb 48 32 00 0f 0b e8 c4 48 32 00 <0f> 0b e9 51 fd ff ff e8 b8 48 32 00 0f 0b e9 82 fd ff ff e8 ac 48
> [  577.826526][ T7216] RSP: 0018:ffffc900073676d8 EFLAGS: 00010246
> [  577.836448][ T7216] RAX: 0000000000000000 RBX: ffff88810d56d1c0 RCX: ffff88810d56d1c0
> [  577.845860][ T7216] RDX: 0000000000000000 RSI: ffff88810d56d1c0 RDI: 0000000000000002
> [  577.856896][ T7216] RBP: ffff888025244700 R08: ffffffff8141a4ec R09: ffffed1004a488ed
> [  577.866712][ T7216] R10: ffff888025244763 R11: ffffed1004a488ec R12: ffff8880660b4c40
> [  577.875736][ T7216] R13: ffff888013930000 R14: ffff888025244700 R15: 0000000000000001
> [  577.889094][ T7216]  ? __mmdrop+0x30c/0x400
> [  577.898466][ T7216]  ? __mmdrop+0x30c/0x400
> [  577.907746][ T7216]  finish_task_switch+0x56f/0x8c0
> [  577.917553][ T7216]  ? __switch_to+0x580/0x1060
> [  577.926962][ T7216]  __schedule+0xa04/0x2310
> [  577.937965][ T7216]  ? firmware_map_remove+0x1a1/0x1a1
> 
> f2-vm login: [  577.947035][ T7216]  ? try_to_wake_up+0x7f3/0x16e0
> [  577.955799][ T7216]  ? preempt_schedule_thunk+0x16/0x18
> [  577.964988][ T7216]  preempt_schedule_common+0x4a/0xc0
> [  577.973670][ T7216]  preempt_schedule_thunk+0x16/0x18
> [  577.985967][ T7216]  try_to_wake_up+0x9eb/0x16e0
> [  577.994498][ T7216]  ? migrate_swap_stop+0x9d0/0x9d0
> [  578.003265][ T7216]  ? rcu_read_lock_held+0xae/0xc0
> [  578.012182][ T7216]  ? rcu_read_lock_sched_held+0xe0/0xe0
> [  578.021280][ T7216]  io_wqe_wake_worker.isra.0+0x4ba/0x670
> [  578.029857][ T7216]  ? io_wq_manager+0xc00/0xc00
> [  578.041295][ T7216]  ? _raw_spin_unlock_irqrestore+0x46/0x50
> [  578.050139][ T7216]  io_wqe_enqueue+0x212/0x980
> [  578.058213][ T7216]  __io_queue_async_work+0x201/0x4a0
> [  578.067518][ T7216]  io_queue_async_work+0x52/0x80
> [  578.078327][ T7216]  __io_queue_sqe+0x986/0xe80
> [  578.086615][ T7216]  ? io_uring_setup+0x3a90/0x3a90
> [  578.094528][ T7216]  ? radix_tree_load_root+0x119/0x1b0
> [  578.102598][ T7216]  ? io_async_task_func+0xa90/0xa90
> [  578.110208][ T7216]  ? __sanitizer_cov_trace_pc+0x1e/0x50
> [  578.120847][ T7216]  io_queue_sqe+0x5e3/0xc40
> [  578.127950][ T7216]  io_submit_sqes+0x17ca/0x26f0
> [  578.135559][ T7216]  ? io_queue_sqe+0xc40/0xc40
> [  578.143129][ T7216]  ? __x64_sys_io_uring_enter+0xa10/0xf00
> [  578.152183][ T7216]  ? xa_store+0x40/0x50
> [  578.162501][ T7216]  ? mutex_lock_io_nested+0x12a0/0x12a0
> [  578.170203][ T7216]  ? do_raw_spin_unlock+0x175/0x260
> [  578.177874][ T7216]  ? _raw_spin_unlock+0x28/0x40
> [  578.185560][ T7216]  ? xa_store+0x40/0x50
> [  578.192755][ T7216]  __x64_sys_io_uring_enter+0xa1b/0xf00
> [  578.201089][ T7216]  ? __io_uring_task_cancel+0x1e0/0x1e0
> [  578.210378][ T7216]  ? __sanitizer_cov_trace_pc+0x1e/0x50
> [  578.218401][ T7216]  ? __audit_syscall_entry+0x3fe/0x540
> [  578.226264][ T7216]  do_syscall_64+0x31/0x70
> [  578.234410][ T7216]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [  578.244957][ T7216] RIP: 0033:0x7f5204b9c89d
> [  578.252372][ T7216] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c3 f5 0c 00 f7 d8 64 89 01 48
> [  578.272398][ T7216] RSP: 002b:00007ffd62bb14e8 EFLAGS: 00000212 ORIG_RAX: 00000000000001aa
> [  578.280966][ T7216] RAX: ffffffffffffffda RBX: 00007ffd62bb1560 RCX: 00007f5204b9c89d
> [  578.289068][ T7216] RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000005
> [  578.300693][ T7216] RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000008
> [  578.308932][ T7216] R10: 0000000000000000 R11: 0000000000000212 R12: 0000000000000001
> [  578.317255][ T7216] R13: 0000000000000000 R14: 00007ffd62bb1520 R15: 0000000000000000
> [  578.328448][ T7216] Kernel Offset: disabled
> [  578.544329][ T7216] Rebooting in 86400 seconds..

I can't get your reproducer to work, and unfortunately that trace doesn't
have some of the debug info? But it looks like it must be the BUG in there.
Can you try with this? Must be related to creds and identity COW'ing,
and you are using multiple processes that share the ring.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 7e35283fc0b1..eb4620ff638e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1501,6 +1501,13 @@ static bool io_grab_identity(struct io_kiocb *req)
 		spin_unlock_irq(&ctx->inflight_lock);
 		req->work.flags |= IO_WQ_WORK_FILES;
 	}
+	if (!(req->work.flags & IO_WQ_WORK_MM) &&
+	    (def->work_flags & IO_WQ_WORK_MM)) {
+		if (id->mm != current->mm)
+			return false;
+		mmgrab(id->mm);
+		req->work.flags |= IO_WQ_WORK_MM;
+	}
 
 	return true;
 }
@@ -1525,13 +1532,6 @@ static void io_prep_async_work(struct io_kiocb *req)
 			req->work.flags |= IO_WQ_WORK_UNBOUND;
 	}
 
-	/* ->mm can never change on us */
-	if (!(req->work.flags & IO_WQ_WORK_MM) &&
-	    (def->work_flags & IO_WQ_WORK_MM)) {
-		mmgrab(id->mm);
-		req->work.flags |= IO_WQ_WORK_MM;
-	}
-
 	/* if we fail grabbing identity, we must COW, regrab, and retry */
 	if (io_grab_identity(req))
 		return;

-- 
Jens Axboe

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