On Wed, Mar 08 2017, Mikulas Patocka wrote:

> On Wed, 8 Mar 2017, NeilBrown wrote:
>> 
>> I don't think this will fix the DM snapshot deadlock by itself.
>> Rather, it make it possible for some internal changes to DM to fix it.
>> The DM change might be something vaguely like:
>> 
>> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
>> index 3086da5664f3..06ee0960e415 100644
>> --- a/drivers/md/dm.c
>> +++ b/drivers/md/dm.c
>> @@ -1216,6 +1216,14 @@ static int __split_and_process_non_flush(struct clone_info *ci)
>> 
>>  	len = min_t(sector_t, max_io_len(ci->sector, ti), ci->sector_count);
>> 
>> +	if (len < ci->sector_count) {
>> +		struct bio *split = bio_split(bio, len, GFP_NOIO, fs_bio_set);
>
> fs_bio_set is a shared bio set, so it is prone to deadlocks. For this 
> change, we would need two bio sets per dm device, one for the split bio 
> and one for the outgoing bio. (this also means having one more kernel 
> thread per dm device)

Yes, two local bio_sets would be best.
But we don't really need those extra kernel threads.  I'll start working
on patches to make them optional, and then to start removing them.

Thanks,
NeilBrown