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

[Mikulas Patocka]

Hi


On Tue, 1 Nov 2022, Eric Biggers wrote:

> 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

I suggest to revert this patch.

> 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.

Propagating dma_alignment through the device mapper stack would be hard 
(because it is not in struct queue_limits). Perhaps we could set 
dma_alignment to be the equivalent to logical_block_size, if the above 
patch could not be reverted - but the we would hit the issue again when 
someone stacks md or other devices over dm.

> 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
> 
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Mikulas