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Re: [Qemu-block] [Qemu-devel] [PATCH v12 2/2] block/backup: fix backup_c

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v12 2/2] block/backup: fix backup_cow_with_offload for last cluster
Date: Thu, 19 Sep 2019 07:02:55 +0000

18.09.2019 23:14, John Snow wrote:
> On 9/17/19 12:07 PM, Vladimir Sementsov-Ogievskiy wrote:
>> We shouldn't try to copy bytes beyond EOF. Fix it.
>> Fixes: 9ded4a0114968e
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> Reviewed-by: Max Reitz <address@hidden>
>> ---
>>   block/backup.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/block/backup.c b/block/backup.c
>> index d8fdbfadfe..89f7f89200 100644
>> --- a/block/backup.c
>> +++ b/block/backup.c
>> @@ -161,7 +161,7 @@ static int coroutine_fn 
>> backup_cow_with_offload(BackupBlockJob *job,
>>       assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
>>       assert(QEMU_IS_ALIGNED(start, job->cluster_size));
>> -    nbytes = MIN(job->copy_range_size, end - start);
>> +    nbytes = MIN(job->copy_range_size, MIN(end, job->len) - start);
> I'm a little confused. I think the patch as written is correct, but I don't 
> know what problem it solves.

last cluster may exceed EOF. And backup_do_cow (who calls  
backup_cow_with_offload) rounds all to clusters.
It's not bad, but we need to crop nbytes before calling actual io functions. 
backup_cow_with_bounce_buffer does the same thing.

> If we're going to allow the caller to pass in an end that's beyond EOF, does 
> that mean we are *requiring* the caller to pass in a value that's aligned?

Actually yes, as we are resetting dirty bitmap.

> We should probably assert what kind of a value we're accepted here, right? We 
> do for start, but should we for 'end' as well?

Yes assertion may be added.

> Then ...
>>       nr_clusters = DIV_ROUND_UP(nbytes, job->cluster_size);
> Don't we just round this right back up immediately anyway? Does that mean we 
> have callers that are passing in an 'end' that's more than 1 job-cluster 
> beyond EOF? That seems like something that should be fixed in the caller, 
> surely?

nr_clusters are used to set/reset dirty bitmap. It's OK. Still, for last 
cluster we can drop it and use nbytes directly. No there should not be such 
nbytes is used to call blk_co_copy_range, and must be cropped to not exceed EOF.

Also, backup_cow_with_bounce_buffer behave in similar way: it crops nbytes.

Of course, there is a place for good refactoring, but I think not in this 
patch, it's a small bug fix.

>>       bdrv_reset_dirty_bitmap(job->copy_bitmap, start,
>>                               job->cluster_size * nr_clusters);

Best regards,

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