Re: Repeatable, raid1+O_DIRECT, hang/warn

[Mikulas Patocka <mpatocka@redhat.com>]

Hi


On Mon, 15 Jun 2026, Vjaceslavs Klimovs wrote:

> Hi Dave, all,
> 
> I'm one of the original reporters and very much a user, not a block/dm
> developer, so please sanity-check all of this.
> 
> Your trace looks like what the two earlier reports hit: a read reaching
> a leaf device with sectors > 0 but phys_seg 0 (an empty bio). One aside
> that may help read the trace: blk_io_trace.error is a __u16, so the
> bracketed values on your C lines are errnos as u16 (65514 = -EINVAL,
> 65531 = -EIO).
> 
> The WARN itself is new, the bad bio isn't. bio_add_page() only started
> rejecting len == 0 in 643893647cac ("block: reject zero length in
> bio_add_page()", v7.1-rc1); on 7.0.8 the same empty bio tripped
> scsi_alloc_sgtables()'s !nr_segs instead, which matches what you saw.
> That fits your "not a recent regression": the condition is older, v7.1
> just made it loud.
> 
> For Tomas's and my reports (QEMU O_DIRECT to the LV block device) the
> origin looks like 5ff3f74e145a ("block: simplify direct io validity
> check", v6.18): blkdev_dio_invalid() now checks only aggregate
> ki_pos | count alignment and dropped the per-segment
> bdev_iter_is_aligned() walk, so a degenerate or misaligned O_DIRECT no
> longer gets -EINVAL at the fops boundary. But your reproducer reads a
> file, which goes through the filesystem O_DIRECT path and never calls
> blkdev_dio_invalid(), and still makes the empty bio. So it isn't only
> that one entry point.

I thought that reverting 5ff3f74e145a and re-introducing the alignment 
check in block/fops.c:blkdev_dio_invalid would fix it - but it wouldn't.

The same problem existed even before 5ff3f74e145a, with the pvmove 
command.

Suppose that the administrator needs to move a logical volume from one 
disk to another and uses pvmove. Pvmove inserts a new dm-mirror target 
underneath the logical volume and uses it to copy the data. Now, the 
dm-mirror target crashes whenever it receives bio with unaligned vectors.

So, I think that the proper way to fix this is to teach dm-mirror/dm-io to 
deal with unaligned bio vectors and handle them properly.

Mikulas