On 09/13/2017 02:26 PM, Eric Blake wrote:
> On 09/13/2017 11:03 AM, Eric Blake wrote:
>> Any device that has request_alignment greater than 512 should be
>> unable to report status at a finer granularity; it may also be
>> simpler for such devices to be guaranteed that the block layer
>> has rounded things out to the granularity boundary (the way the
>> block layer already rounds all other I/O out).  Besides, getting
>> the code correct for super-sector alignment also benefits us
>> for the fact that our public interface now has byte granularity,
>> even though none of our drivers have byte-level callbacks.
>>
>> Add an assertion in blkdebug that proves that the block layer
>> never requests status of unaligned sections, similar to what it
>> does on other requests (while still keeping the generic helper
>> in place for when future patches add a throttle driver).  Note
>> that iotest 177 already covers this (it would fail if you use
>> just the blkdebug.c hunk without the io.c changes).  Meanwhile,
>> we can drop assertions in callers that no longer have to pass
>> in sector-aligned addresses.
> 
> Bummer - 'git bisect' says this patch causes iotests 190 to hang.  I'm
> investigating root cause, but I'll have to post a fixup once I figure it
> out.

Found it:

> +    /* Round out to request_alignment boundaries */
> +    align = MAX(bs->bl.request_alignment, BDRV_SECTOR_SIZE);
> +    aligned_offset = QEMU_ALIGN_DOWN(offset, align);
> +    aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset;

ROUND_UP(64-bit, 32-bit) has a subtle bug: it truncates the operation at
32 bits, instead of producing a 64-bit result.  Using QEMU_ROUND_UP
instead does NOT have the bug.

That's a ticking time bomb, so I'll patch ROUND_UP() directly as a
pre-requisite, then reply to the cover letter with a Based-on tag.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org