Re: [Qemu-devel] [PATCH v5 4/6] nbd/server: implement dirty bitmap export

[Eric Blake <eblake@redhat.com>]

On 06/20/2018 10:43 AM, Eric Blake wrote:
> On 06/20/2018 06:24 AM, Eric Blake wrote:
> 
>>> +/* Set several extents, describing region of given @length with 
>>> given @flags.
>>> + * Do not set more than @nb_extents, return number of set extents.
>>> + */
>>> +static unsigned add_extents(NBDExtent *extents, unsigned nb_extents,
>>> +                            uint64_t length, uint32_t flags)
>>> +{
>>> +    unsigned i = 0;
>>> +    uint32_t max_extent = QEMU_ALIGN_DOWN(INT32_MAX, BDRV_SECTOR_SIZE);
>>
>> This is too small of a granularity wrong when the server advertised 4k 
>> alignment during NBD_OPT_GO; it should probably refer to 
>> bs->bl.request_alignment.
> 
> In fact, we can just use INT32_MAX. The dirty bitmap has a granularity 
> at least as large as the sector size, but no smaller than the 
> request_alignment. We don't have to worry about alignment here, as the 
> extents will already be naturally aligned when converting from the 
> bitmap into extents in the caller.

Oh, I see. The NBD protocol can only ask for a length of up to 32 bits, 
but if you learn via bitmap query that the entire rest of the image has 
the same state, you can indeed call add_extents() with a value larger 
than 32 bits, that needs to be fragmented back down into sub-32-bit 
chunks.  INT32_MAX isn't quite right, but rounding may still pick the 
wrong granularity on an export with 4k alignment; however, INT32_MAX + 
1U is just fine, and unlikely to run into alignment issues.

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