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Re: [Qemu-block] [Qemu-devel] [PATCH v12 2/2] block/backup: fix backup_c
Re: [Qemu-block] [Qemu-devel] [PATCH v12 2/2] block/backup: fix backup_cow_with_offload for last cluster
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,
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);