Re: [Qemu-devel] [PATCH v6 05/18] dirty-bitmap: Change bdrv_dirty_bitmap_size() to report bytes

[Eric Blake <eblake@redhat.com>]

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On 09/08/2017 07:22 AM, Kevin Wolf wrote:
> Am 30.08.2017 um 23:05 hat Eric Blake geschrieben:
>> We are still using an internal hbitmap that tracks a size in sectors,
>> with the granularity scaled down accordingly, because it lets us
>> use a shortcut for our iterators which are currently sector-based.
>> But there's no reason we can't track the dirty bitmap size in bytes,
>> since it is (mostly) an internal-only variable (remember, the size
>> is how many bytes are covered by the bitmap, not how many bytes the
>> bitmap occupies).  Furthermore, we're already reporting bytes for
>> bdrv_dirty_bitmap_granularity(); mixing bytes and sectors in our
>> return values is a recipe for confusion.  A later cleanup will
>> convert dirty bitmap internals to be entirely byte-based,
>> eliminating the intermediate sector rounding added here; and
>> technically, since bdrv_getlength() already rounds up to sectors,
>> our use of DIV_ROUND_UP is more for theoretical completeness than
>> for any actual rounding.
>>
>> The only external caller in qcow2-bitmap.c is temporarily more verbose
>> (because it is still using sector-based math), but will later be
>> switched to track progress by bytes instead of sectors.
>>
>> Use is_power_of_2() while at it, instead of open-coding that, and
>> add an assertion where bdrv_getlength() should not fail.
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>> Reviewed-by: John Snow <jsnow@redhat.com>
> 
> I think I would have preferred to change the unit of
> BdrvDirtyBitmap.size in one patch and the unit of the return value of
> bdrv_dirty_bitmap_size() in another one to keep review a bit easier.

I can split on respin, if there's still enough reason for a respin.

>> @@ -305,13 +307,14 @@ BdrvDirtyBitmap *bdrv_reclaim_dirty_bitmap(BlockDriverState *bs,
>>  void bdrv_dirty_bitmap_truncate(BlockDriverState *bs)
>>  {
>>      BdrvDirtyBitmap *bitmap;
>> -    uint64_t size = bdrv_nb_sectors(bs);
>> +    int64_t size = bdrv_getlength(bs);
>>
>> +    assert(size >= 0);
> 
> How can you assert that there will never be an error? Even if it's
> correct (I don't know whether you can have dirty bitmaps on devices that
> don't use the cached value), this needs at least a comment.

The old code wasn't checking for errors; if an error occurs, we have no
way to report it. So I indeed need to audit whether all callers have a
cached length at this point in time (it can't fail), or else change
bdrv_dirty_bitmap_truncate() to be able to fail (pass failure along) and
update all callers.  This may indeed be reason for a respin, depending
on what I find.

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


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