Re: [PATCH] block: use static bio_set for bio_split() calls

[Hannes Reinecke <hare@suse.de>]

On 4/25/19 5:36 PM, Ming Lei wrote:
> On Thu, Apr 25, 2019 at 04:32:42PM +0200, Hannes Reinecke wrote:
>> On 4/25/19 2:41 AM, Ming Lei wrote:
>>> On Thu, Apr 25, 2019 at 06:14:22AM +0800, Ming Lei wrote:
>>>> On Wed, Apr 24, 2019 at 10:20:46AM -0700, Sagi Grimberg wrote:
>>>>>
>>>>>> per-queue bioset is used originally for avoiding deadlock, are you
>>>>>> sure the static bioset is safe?
>>>>>
>>>>> Can you explain this? I didn't find any indication of that in the change
>>>>> log history...
>>>>>
>>>>> Originally introduced by Kent:
>>>>
>>>> bio split can be run from stacking drivers, for example, MD over NVMe,
>>>> if the global reserved mempool is consumed by MD bio splitting, then
>>>> no any progress can be made when splitting on bio submitted to NVMe.
>>>>
>>>> Kent may have more details...
>>>
>>> I guess it might be fine to use one shared global bio_set for all
>>> lowest underlying queues, could be all queues except for loop, dm, md
>>> , drbd, bcache, ...
>>>
>> But wasn't the overall idea of stacking drivers that we propagate the queue
>> limits up to the uppermost drivers, so that we have to do a split only at
>> the upper layers?
> 
> For example, LVM over RAID, the limits of LVM queue is figured out and fixed
> during creating LVM. However, new device may be added to the RAID. Then the
> underlying queue's limit may not be propagated to LVM's queue's limit.
> 
> And we did discuss the topic of 'block: dm: restack queue_limits'
> before, looks not see any progress made.
> 
> Also loop doesn't consider stack limits at all.
> 
>> Furthermore, it's not every bio which needs to be split, only those which
>> straddle some device limitations.
>> The only ones not being able to propagate the queue limits is MD, and that
>> is already using a private bio_set here.
> 
> If DM and the lowest queue share one same bio_set(mem_pool), it isn't
> enough for MD to use private bio_set.
> 
Ah, right. I've reviewed the patches implementing the per-queue biosets, 
and indeed we'll need to use them.
But meanwhile I've found another way of circumventing this issue, so 
this patch can be dropped.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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