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

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On Wed, Nov 02, 2022 at 02:45:10PM -0400, Mikulas Patocka wrote:
> 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@xxxxxxxxxx>
> >     Date:   Fri Jun 10 12:58:29 2022 -0700
> > 
> >         block: relax direct io memory alignment
> 
> I suggest to revert this patch.

I hope we can make that option a last resort.
 
> > 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.

It looks straight forward to relocate the attribute to the the
queue_limits. If it stacks correctly, then no one would encounter a
problem no matter what md/dm combination you have.

I have something that looks promising, but I'm trying to give it a
thorough test before sending out another incomplete patch. Hopefully
ready by end of the day.



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