[dm-devel] Regression: wrong DIO alignment check with dm-crypt

[Eric Biggers]

Hi,

I happened to notice the following QEMU bug report:

https://gitlab.com/qemu-project/qemu/-/issues/1290

I believe it's a regression from the following kernel commit:

    commit b1a000d3b8ec582da64bb644be633e5a0beffcbf
    Author: Keith Busch <kbusch at kernel.org>
    Date:   Fri Jun 10 12:58:29 2022 -0700

        block: relax direct io memory alignment

The bug is that if a dm-crypt device is set up with a crypto sector size (and
thus also a logical_block_size) of 4096, then the block layer now lets through
direct I/O requests to dm-crypt when the user buffer has only 512-byte
alignment, instead of the 4096-bytes expected by dm-crypt in that case.  This is
because the dma_alignment of the device-mapper device is only 511 bytes.

This has two effects in this case:

    - The error code for DIO with a misaligned buffer is now EIO, instead of
      EINVAL as expected and documented.  This is because the I/O reaches
      dm-crypt instead of being rejected by the block layer.

    - STATX_DIOALIGN reports 512 bytes for stx_dio_mem_align, instead of the
      correct value of 4096.  (Technically not a regression since STATX_DIOALIGN
      is new in v6.1, but still a bug.)

Any thoughts on what the correct fix is here?  Maybe the device-mapper layer
needs to set dma_alignment correctly?  Or maybe the block layer needs to set it
to 'logical_block_size - 1' by default?

- Eric