On 29.11.19 13:01, Vladimir Sementsov-Ogievskiy wrote:
> 11.11.2019 19:02, Max Reitz wrote:
>> While bdrv_replace_node() will not follow through with it, a specific
>> @replaces asks the mirror job to create a loop.
>>
>> For example, say both the source and the target share a child where the
>> source is a filter; by letting @replaces point to the common child, you
>> ask for a loop.
>>
>> Or if you use @replaces in drive-mirror with sync=none and
>> mode=absolute-paths, you generally ask for a loop (@replaces must point
>> to a child of the source, and sync=none makes the source the backing
>> file of the target after the job).
>>
>> bdrv_replace_node() will not create those loops, but by doing so, it
>> ignores the user-requested configuration, which is not ideally either.
>> (In the first example above, the target's child will remain what it was,
>> which may still be reasonable.  But in the second example, the target
>> will just not become a child of the source, which is precisely what was
>> requested with @replaces.)
>>
>> So prevent such configurations, both before the job, and before it
>> actually completes.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block.c                   | 30 ++++++++++++++++++++++++
>>   block/mirror.c            | 19 +++++++++++++++-
>>   blockdev.c                | 48 ++++++++++++++++++++++++++++++++++++++-
>>   include/block/block_int.h |  3 +++
>>   4 files changed, 98 insertions(+), 2 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index 0159f8e510..e3922a0474 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -6259,6 +6259,36 @@ out:
>>       return to_replace_bs;
>>   }
>>   
>> +/*
>> + * Return true iff @child is a (recursive) child of @parent, with at
>> + * least @min_level edges between them.
>> + *
>> + * (If @min_level == 0, return true if @child == @parent.  For
>> + * @min_level == 1, @child needs to be at least a real child; for
>> + * @min_level == 2, it needs to be at least a grand-child; and so on.)
>> + */
>> +bool bdrv_is_child_of(BlockDriverState *child, BlockDriverState *parent,
>> +                      int min_level)
>> +{
>> +    BdrvChild *c;
>> +
>> +    if (child == parent && min_level <= 0) {
>> +        return true;
>> +    }
>> +
>> +    if (!parent) {
>> +        return false;
>> +    }
>> +
>> +    QLIST_FOREACH(c, &parent->children, next) {
>> +        if (bdrv_is_child_of(child, c->bs, min_level - 1)) {
>> +            return true;
>> +        }
>> +    }
>> +
>> +    return false;
>> +}
>> +
>>   /**
>>    * Iterates through the list of runtime option keys that are said to
>>    * be "strong" for a BDS.  An option is called "strong" if it changes
>> diff --git a/block/mirror.c b/block/mirror.c
>> index 68a4404666..b258c7e98b 100644
>> --- a/block/mirror.c
>> +++ b/block/mirror.c
>> @@ -701,7 +701,24 @@ static int mirror_exit_common(Job *job)
>>            * there.
>>            */
>>           if (bdrv_recurse_can_replace(src, to_replace)) {
>> -            bdrv_replace_node(to_replace, target_bs, &local_err);
>> +            /*
>> +             * It is OK for @to_replace to be an immediate child of
>> +             * @target_bs, because that is what happens with
>> +             * drive-mirror sync=none mode=absolute-paths: target_bs's
>> +             * backing file will be the source node, which is also
>> +             * to_replace (by default).
>> +             * bdrv_replace_node() handles this case by not letting
>> +             * target_bs->backing point to itself, but to the source
>> +             * still.
>> +             */
> 
> Hmm.. So, we want the following valid case:
> 
> (other parents of source) ----> source = to_replace <--- backing --- target
> 
> becomes
> 
> (other parents of source) ----> target --- backing ---> source
> 
> But it seems for me, that the following is not less valid:
> 
> (other parents of source) ----> source = to_replace <--- backing --- X <--- backing --- target
> 
> becomes
> 
> (other parents of source) ----> target --- backing ---> X --- backing ---> source

I think it is less valid.  The first case works with sync=none, because
target is initially empty and then you just copy all new data, so the
target keeps looking like the source.

But in the second case, there are intermediate nodes that mean that
target may well not look like the source.

(Yes, you have the same problem if you use sync=none or sync=full to a
completely independent node.  But that still means that while the first
case is always valid, the second may be problematic.)

> And what we actually want to prevent, is when to_replace is not source, but child (may be not direct)
> of source..
> 
> Also, with your check you still allow silent no-change in the following case:
> 
> source --- backing --> to_replace <-- backing -- target

You mean if source is a filter on to_replace?  (Because otherwise you
can’t replace that node.)

Is that really a no-change?  Shouldn’t we get

source --> target --> to_replace

?  (And what else would you expect?)

So maybe we don’t want to prevent that, because I think it can make sense.

Max

> ====
> 
> In other words, replacing make sense, only if to_replace has some other parents, which are not
> children (may be not direct) of target.. And the only known such case is when in the same time
> to_replace == source.
> 
> so, shouldn't the following be
> 
> if (to_replace == src || !bdrv_is_child_of(to_replace, target_bs, 1) {
> 
> or, may be, to allow also replace filters above src, keeping backing link :
> 
> if (bdrv_is_child_of(src, to_replace, 0) || !bdrv_is_child_of(to_replace, target_bs, 1) {
> 
>> +            if (!bdrv_is_child_of(to_replace, target_bs, 2)) {
>> +                bdrv_replace_node(to_replace, target_bs, &local_err);
>> +            } else {
>> +                error_setg(&local_err, "Can no longer replace '%s' by '%s', "
>> +                           "because the former is now a child of the latter, "
>> +                           "and doing so would thus create a loop",
>> +                           to_replace->node_name, target_bs->node_name);
>> +            }
>>           } else {
>>               error_setg(&local_err, "Can no longer replace '%s' by '%s', "
>>                          "because it can no longer be guaranteed that doing so "
>> diff --git a/blockdev.c b/blockdev.c
>> index 9dc2238bf3..d29f147f72 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -3824,7 +3824,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>       }
>>   
>>       if (has_replaces) {
>> -        BlockDriverState *to_replace_bs;
>> +        BlockDriverState *to_replace_bs, *target_backing_bs;
>>           AioContext *replace_aio_context;
>>           int64_t bs_size, replace_size;
>>   
>> @@ -3839,6 +3839,52 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>               return;
>>           }
>>   
>> +        if (bdrv_is_child_of(to_replace_bs, target, 1)) {
>> +            error_setg(errp, "Replacing %s by %s would result in a loop, "
>> +                       "because the former is a child of the latter",
>> +                       to_replace_bs->node_name, target->node_name);
>> +            return;
>> +        }
>> +
>> +        if (backing_mode == MIRROR_SOURCE_BACKING_CHAIN ||
>> +            backing_mode == MIRROR_OPEN_BACKING_CHAIN)
>> +        {
>> +            /*
>> +             * While we do not quite know what OPEN_BACKING_CHAIN
>> +             * (used for mode=existing) will yield, it is probably
>> +             * best to restrict it exactly like SOURCE_BACKING_CHAIN,
>> +             * because that is our best guess.
>> +             */
>> +            switch (sync) {
>> +            case MIRROR_SYNC_MODE_FULL:
>> +                target_backing_bs = NULL;
>> +                break;
>> +
>> +            case MIRROR_SYNC_MODE_TOP:
>> +                target_backing_bs = backing_bs(bs);
>> +                break;
>> +
>> +            case MIRROR_SYNC_MODE_NONE:
>> +                target_backing_bs = bs;
>> +                break;
>> +
>> +            default:
>> +                abort();
>> +            }
>> +        } else {
>> +            assert(backing_mode == MIRROR_LEAVE_BACKING_CHAIN);
>> +            target_backing_bs = backing_bs(target);
>> +        }
>> +
>> +        if (bdrv_is_child_of(to_replace_bs, target_backing_bs, 0)) {
>> +            error_setg(errp, "Replacing '%s' by '%s' with this sync mode would "
>> +                       "result in a loop, because the former would be a child "
>> +                       "of the latter's backing file ('%s') after the mirror "
>> +                       "job", to_replace_bs->node_name, target->node_name,
>> +                       target_backing_bs->node_name);
>> +            return;
>> +        }
>> +
>>           replace_aio_context = bdrv_get_aio_context(to_replace_bs);
>>           aio_context_acquire(replace_aio_context);
>>           replace_size = bdrv_getlength(to_replace_bs);
>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>> index 589a797fab..7064a1a4fa 100644
>> --- a/include/block/block_int.h
>> +++ b/include/block/block_int.h
>> @@ -1266,6 +1266,9 @@ void bdrv_format_default_perms(BlockDriverState *bs, BdrvChild *c,
>>   bool bdrv_recurse_can_replace(BlockDriverState *bs,
>>                                 BlockDriverState *to_replace);
>>   
>> +bool bdrv_is_child_of(BlockDriverState *child, BlockDriverState *parent,
>> +                      int min_level);
>> +
>>   /*
>>    * Default implementation for drivers to pass bdrv_co_block_status() to
>>    * their file.
>>
> 
>