Re: Commit 'iomap: add support for dma aligned direct-io' causes qemu/KVM boot failures

[Maxim Levitsky <mlevitsk@redhat.com>]

On Sun, 2022-10-02 at 07:56 -0600, Keith Busch wrote:
> On Sun, Oct 02, 2022 at 11:59:42AM +0300, Maxim Levitsky wrote:
> > On Thu, 2022-09-29 at 19:35 +0200, Paolo Bonzini wrote:
> > > On 9/29/22 18:39, Christoph Hellwig wrote:
> > > > On Thu, Sep 29, 2022 at 10:37:22AM -0600, Keith Busch wrote:
> > > > > > I am aware, and I've submitted the fix to qemu here:
> > > > > > 
> > > > > >   https://lists.nongnu.org/archive/html/qemu-block/2022-09/msg00398.html
> > > > > 
> > > > > I don't think so. Memory alignment and length granularity are two completely
> > > > > different concepts. If anything, the kernel's ABI had been that the length
> > > > > requirement was also required for the memory alignment, not the other way
> > > > > around. That usage will continue working with this kernel patch.
> > 
> > Yes, this is how I also understand it - for example for O_DIRECT on a file which
> > resides on 4K block device, you have to use page aligned buffers.
> > 
> > But here after the patch, 512 aligned buffer starts working as well - If I
> > understand you correctly the ABI didn't guarantee that such usage would fail,
> > but rather that it might fail.
> 
> The kernel patch will allow buffer alignment to work with whatever the hardware
> reports it can support. It could even as low as byte aligned if that's the
> hardware can use that.
> 
> The patch aligns direct-io with the same criteria blk_rq_map_user() has always
> used to know if the user space buffer is compatible with the hardware's dma
> requirements. Prior to this patch, the direct-io memory alignment was an
> artificial software constraint, and that constraint creates a lot of
> unnecessary memory pressure.
> 
> As has always been the case, each segment needs to be a logical block length
> granularity. QEMU assumed a buffer's page offset also defined the logical block
> size instead of using the actual logical block size that it had previously
> discovered directly.
> 
> > If I understand that correctly, after the patch in question, 
> > qemu is able to use just 512 bytes aligned buffer to read a single 4K block from the disk,
> > which supposed to fail but wasn't guarnteed to fail.
> > 
> > Later qemu it submits iovec which also reads a 4K block but in two parts,
> > and if I understand that correctly, each part (iov) is considered
> > to be a separate IO operation,  and thus each has to be in my case 4K in size, 
> > and its memory buffer *should* also be 4K aligned.
> > 
> > (but it can work with smaller alignement as well).
> 
> Right. The iov length needs to match the logical block size. The iov's memory
> offset needs to align to the queue's dma_alignment attribute. The memory
> alignment may be smaller than a block size.
>  
> > Assuming that I understand all of this correctly, I agree with Paolo that this is qemu
> > bug, but I do fear that it can cause quite some problems for users,
> > especially for users that use outdated qemu version.
> > 
> > It might be too much to ask, but maybe add a Kconfig option to keep legacy behavier
> > for those that need it?
> 
> Kconfig doesn't sound right.
> 
> The block layer exports all the attributes user space needs to know about for
> direct io.
> 
>   iov length:    /sys/block/<block-dev>/queue/logical_block_size
>   iov mem align: /sys/block/<block-dev>/queue/dma_alignment
> 
> If you really want to change the behavior, I think maybe we could make the
> dma_alignment attribute writeable (or perhaps add a new attribute specifically
> for dio_alignment) so the user can request something larger.
> 
All makes sense now. 

New attribute could make sense I guess, and can be set by an udev rule or something.


Anyway I won't worry about this for now, and if there are issues I'll see how we could work
around them.

Thanks for everything,
Best regards,
	Maxim Levitsky