Re: [PATCH] ceph: fix potential stray locked folios during umount

[Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>]

On Sat, 2026-04-18 at 21:39 +0800, Li Lei wrote:
> During umount, we only wait for stopping_blockers to drop to zero for
> a certain time specified by mount_timeout, and continue the rest of
> the procedure even if there are inflight requests. This behavior may
> leave some folios locked even after the cephfs umounted, which causes
> other kernel threads to hung.
> 
> Buffered read process calls filemap_update_page() and waits on
> folio_put_wait_locked() with TASK_KILLABLE flag set, which means this
> process could be killed and the filesystem could be umount successfully
> (no file opened in it). Umount calls truncate_inode_pages() and waits
> on locked pages for those inodes whose i_count == 0. In these way,
> there would be no locked folios for this filesystem left in system
> after umount exits.
> 
> However, things are different for cephfs. Cephfs calls ihold() and
> submits osd request for buffered read and gets folio locked. Once the
> buffered read process is killed, the inode will be skipped in
> evict_inodes(), because its i_count > 0. Forthemore, the folios are
> still locked. It can only be unlocked in netfs_unlock_read_folio().
> 
> stopping_blocks should block umount from proceeding, but it only waits
> for mount_timeout (default 60s) even if there are still flying request
> out there, leaving stray locked folios. Other kthread, like kcompactd
> , could be stuck on those locked folioes forever.
> 
> Steps to Reproduce:
> 1. echo 3 > /proc/sys/vm/drop_caches.
> 2. dd if=cephfs/xxx.img of=/dev/null
>     Make sure cephfs/xxx.img is big enough to make time for us to do the
>     following command
> 3. execute 'systemctl stop ceph-osd@*' on the osd nodes
>     It would be great if you have a tiny cluster. Stopping all the osds
>     would be much easier.
> 4. kill -9 `pidof dd`.
>     Buffered read process must be killed at that moment. But inflight
>     read requests can be observed in the /sys/kernel/debug/ceph/xxxx/osdc
> 5. umount cephfs
>     Wait for 60s if you mount cephfs by using the default mount option.
> 
> We got the warning:
> ceph: [b2c9a006-9ad8-48e9-8257-6fb1e1b91014 66562]: umount timed out, 0
> VFS: Busy inodes after unmount of ceph (ceph)
> 
> if check_data_corruption option disable, kcompactd may stuck in the
> future. If it is eanbled, we catch the bug immediately.
> 
> [94543.042953] ------------[ cut here ]------------
> [94543.049391] kernel BUG at fs/super.c:654!
> [94543.054171] Oops: invalid opcode: 0000 [#1] SMP PTI
> [94543.059881] CPU: 25 UID: 0 PID: 3451674 Comm: umount Kdump: loaded Tainted: G S         OE       7.0.0-dirty #2 PREEMPTLAZY
> [94543.072678] Tainted: [S]=CPU_OUT_OF_SPEC, [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
> [94543.080918] Hardware name: Dell Inc. PowerEdge R730xd/072T6D, BIOS 2.5.5 08/16/2017
> [94543.089755] RIP: 0010:generic_shutdown_super+0x111/0x120
> [94543.095982] Code: cc cc e8 c2 1f ef ff 48 8b bb d0 00 00 00 eb db 48 8b 43 28 48 8d b3 98 03 00 00 48 c7 c7 90 09 55 8c 48 8b 10 e8 0f c3 cc ff <0f> 0b 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 90 90 90 90 90 90 90
> [94543.117607] RSP: 0018:ffffce35f8c53d40 EFLAGS: 00010246
> [94543.123793] RAX: 000000000000002d RBX: ffff8ba94d0d9000 RCX: 0000000000000000
> [94543.132125] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8bc0df91c600
> [94543.140460] RBP: ffffffffc13b52c0 R08: 0000000000000000 R09: ffffce35f8c53be0
> [94543.148801] R10: 0000000000000001 R11: 0000000000000001 R12: ffff8ba94af0e000
> [94543.157150] R13: ffff8ba94d0d9000 R14: 0000000000000004 R15: ffff8ba946d9a000
> [94543.165505] FS:  00007fb1c607c840(0000) GS:ffff8bc1520e4000(0000) knlGS:0000000000000000
> [94543.174943] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [94543.181769] CR2: 00000000d64e5000 CR3: 000000189c9f2002 CR4: 00000000003726f0
> [94543.190160] Call Trace:
> [94543.193317]  <TASK>
> [94543.196088]  kill_anon_super+0x12/0x40
> [94543.200719]  ceph_kill_sb+0xda/0x2c0 [ceph]
> [94543.205877]  ? radix_tree_delete_item+0x68/0xd0
> [94543.211395]  deactivate_locked_super+0x31/0xb0
> [94543.216815]  cleanup_mnt+0xcb/0x110
> [94543.221169]  task_work_run+0x58/0x80
> [94543.225629]  exit_to_user_mode_loop+0x13f/0x4d0
> [94543.231163]  do_syscall_64+0x1ef/0x840
> [94543.235827]  ? do_syscall_64+0x101/0x840
> [94543.240687]  ? do_user_addr_fault+0x20e/0x6b0
> [94543.246036]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> [94543.252166] RIP: 0033:0x7fb1c5f0ccab
> [94543.256650] Code: 73 31 0e 00 f7 d8 64 89 01 48 83 c8 ff c3 90 f3 0f 1e fa 31 f6 e9 05 00 00 00 0f 1f 44 00 00 f3 0f 1e fa b8 a6 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 05 c3 0f 1f 40 00 48 8b 15 39 31 0e 00 f7 d8
> [94543.278690] RSP: 002b:00007ffe96f80ea8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
> [94543.287710] RAX: 0000000000000000 RBX: 00007fb1c61fb264 RCX: 00007fb1c5f0ccab
> [94543.296251] RDX: fffffffffffffe88 RSI: 0000000000000000 RDI: 000055ae9dcc6ec0
> [94543.304801] RBP: 000055ae9dcc6c90 R08: 0000000000000000 R09: 00007ffe96f7fc50
> [94543.313358] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> [94543.321922] R13: 000055ae9dcc6ec0 R14: 000055ae9dcc6da0 R15: 0000000000000000
> 
> So make it wait until all the flying requests returns for clean and safe
> umount.
> 
> Fixes: 1464de9f813e ("ceph: wait for OSD requests' callbacks to finish when unmounting")
> Signed-off-by: Li Lei <lilei24@kuaishou.com>
> ---
>  fs/ceph/super.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ceph/super.c b/fs/ceph/super.c
> index 2aed6b3..48e63c1 100644
> --- a/fs/ceph/super.c
> +++ b/fs/ceph/super.c
> @@ -1569,13 +1569,10 @@ static void ceph_kill_sb(struct super_block *s)
>  	spin_unlock(&mdsc->stopping_lock);
>  
>  	if (wait && atomic_read(&mdsc->stopping_blockers)) {
> -		long timeleft = wait_for_completion_killable_timeout(
> -					&mdsc->stopping_waiter,
> -					fsc->client->options->mount_timeout);
> -		if (!timeleft) /* timed out */
> -			pr_warn_client(cl, "umount timed out, %ld\n", timeleft);
> -		else if (timeleft < 0) /* killed */
> -			pr_warn_client(cl, "umount was killed, %ld\n", timeleft);
> +		int rc = wait_for_completion_killable(
> +					&mdsc->stopping_waiter);
> +		if (rc < 0) /* killed */
> +			pr_warn_client(cl, "umount was killed\n");
>  	}
>  
>  	mdsc->stopping = CEPH_MDSC_STOPPING_FLUSHED;

I am not completely sure that it's right approach to fix the problem. If I
understood correctly, we have situation that somehow OSD is down and dd process
has been killed. It's not normal and usual situation. So, your suggestion is to
hung in unmount process forever because we cannot service the flying requests.
Is it really good way to fix the problem? Probably, we need to have more smart
logic here of waiting flying requests ending. But timeout makes sense to prevent
from the situation that something is going wrong and we cannot finish unmount at
all. What do you think? Maybe we need to have some loop that checks the state by
waking up after some timeout. If the number of flying requests is decreasing,
then we should wait. But if nothing is changing with time, then it means that
something is wrong and it makes sense to unmount anyway. Because, finally, we
will have some call trace in system log, anyway. Does it make sense?

Also, we have likewise pattern in the ceph_kill_sb():

	if (atomic64_read(&mdsc->dirty_folios) > 0) {
		wait_queue_head_t *wq = &mdsc->flush_end_wq;
		long timeleft = wait_event_killable_timeout(*wq,
					atomic64_read(&mdsc->dirty_folios) <=
0,
					fsc->client->options->mount_timeout);
		if (!timeleft) /* timed out */
			pr_warn_client(cl, "umount timed out, %ld\n",
timeleft);
		else if (timeleft < 0) /* killed */
			pr_warn_client(cl, "umount was killed, %ld\n",
timeleft);
	}

Do we need to do something here too?

Thanks,
Slava.