Re: Repeatable, raid1+O_DIRECT, hang/warn

[Keith Busch <kbusch@kernel.org>]

On Mon, Jun 15, 2026 at 09:20:23AM -0600, Keith Busch wrote:
> On Sun, Jun 14, 2026 at 05:57:48PM +0000, Dr. David Alan Gilbert wrote:
> > Jun 14 18:08:32 dalek kernel: device-mapper: raid1: Mirror read failed from 252:24. Trying alternative device.
> > Jun 14 18:08:32 dalek kernel: ------------[ cut here ]------------
> > Jun 14 18:08:32 dalek dmeventd[1010]: Primary mirror device 252:24 read failed.
> > Jun 14 18:08:32 dalek kernel: WARNING: block/bio.c:1044 at bio_add_page+0x18b/0x250, CPU#15: kworker/15:1/369
> > Jun 14 18:08:32 dalek dmeventd[1010]: main-lvol0 is now in-sync.
> > Jun 14 18:08:32 dalek kernel: Modules linked in: nft_masq nft_reject_ipv4 act_csum cls_u32 sch_htb nf_nat_tftp nf_conntrack_tftp bridge stp llc rfkill nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reje>
> > Jun 14 18:08:32 dalek kernel:  drm_panel_backlight_quirks gpu_sched drm_suballoc_helper video nvme drm_display_helper nvme_core cec nvme_keyring sp5100_tco nvme_auth wmi serio_raw fuse scsi_dh_alua i2c_dev scsi_dh_rdac scsi_dh_emc
> > Jun 14 18:08:32 dalek kernel: CPU: 15 UID: 0 PID: 369 Comm: kworker/15:1 Not tainted 7.1.0-rc7+ #786 PREEMPT(lazy) 
> > Jun 14 18:08:32 dalek kernel: Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./X570 Pro4, BIOS P3.10 07/13/2020
> > Jun 14 18:08:32 dalek kernel: Workqueue: kmirrord do_mirror
> > Jun 14 18:08:32 dalek kernel: RIP: 0010:bio_add_page+0x18b/0x250
> > Jun 14 18:08:32 dalek kernel: Code: 24 10 4c 8b 04 24 84 c0 0f 85 c9 00 00 00 41 0f b7 40 78 48 8b 74 24 08 8b 4c 24 14 e9 b4 fe ff ff 0f 0b 31 c0 e9 55 d1 af 00 <0f> 0b eb f5 48 8b 7f 08 83 7f 60 05 0f 85 00 ff ff ff 49 8b 3b 4c
> > Jun 14 18:08:32 dalek kernel: RSP: 0018:ffffd1fb8176fc10 EFLAGS: 00010246
> > Jun 14 18:08:32 dalek kernel: RAX: 0000000000000000 RBX: ffffd1fb8176fd18 RCX: 0000000000000000
> > Jun 14 18:08:32 dalek kernel: RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8d1a8eb28b00
> > Jun 14 18:08:32 dalek kernel: RBP: 0000000000000000 R08: ffffd1fb8176fc38 R09: ffffd1fb8176fc40
> > Jun 14 18:08:32 dalek kernel: R10: ffffd1fb8176fc34 R11: 0000000000000000 R12: 0000000000000000
> > Jun 14 18:08:32 dalek kernel: R13: ffffd1fb8176fd90 R14: 0000000000000001 R15: ffff8d1a8eb28b00
> > Jun 14 18:08:32 dalek kernel: FS:  0000000000000000(0000) GS:ffff8d29d161f000(0000) knlGS:0000000000000000
> > Jun 14 18:08:32 dalek kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > Jun 14 18:08:32 dalek kernel: CR2: 00007f0ddcd7b9d0 CR3: 000000023dcbf000 CR4: 0000000000350ef0
> > Jun 14 18:08:32 dalek kernel: Call Trace:
> > Jun 14 18:08:32 dalek kernel:  <TASK>
> > Jun 14 18:08:32 dalek kernel:  do_region+0x227/0x2a0
> 
> I think the problem is that do_region is tracking the "remaining" in
> sector granularity, but devices can have dma alignment such that it's
> valid to have sub-sector vectors. Rounding the length appended
> to_sectors() creates a 0 length subtraction, so the loop thinks no
> progress is made and loops forever. If we track it in bytes instead of
> sectors, then that should fix this observation.

I recreated your observation and this patch below appears to fix the
stuck behavior.

---
diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
index 1db565b376200..d72b9331c2fd1 100644
--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -362,19 +362,26 @@ static void do_region(const blk_opf_t opf, unsigned int region,
                        bio->bi_iter.bi_size = num_sectors << SECTOR_SHIFT;
                        remaining -= num_sectors;
                } else {
-                       while (remaining) {
+                       unsigned long byte_remaining = to_bytes(remaining);
+
+                       while (byte_remaining) {
                                /*
                                 * Try and add as many pages as possible.
                                 */
                                dp->get_page(dp, &page, &len, &offset);
-                               len = min(len, to_bytes(remaining));
+                               len = min(len, byte_remaining);
                                if (!bio_add_page(bio, page, len, offset))
                                        break;

                                offset = 0;
-                               remaining -= to_sector(len);
+                               byte_remaining -= len;
                                dp->next_page(dp);
                        }
+                       remaining = to_sector(byte_remaining);
                }

                atomic_inc(&io->count);
--