Re: Deadlock possibly caused by too_many_isolated.

[Wu Fengguang]

> > We surely need 1 set aside for each level of that stack that will
> > potentially consume one. 1 should be enough for the generic pool, and
> > then clones will use a separate pool. So md and friends should really
> > have a pool per device, so that stacking will always work properly.
> 
> Agreed for the deadlock problem.
> 
> > There should be no throughput concerns, it should purely be a safe guard
> > measure to prevent us deadlocking when doing IO for reclaim.
> 
> It's easy to verify whether the minimal size will have negative
> impacts on IO throughput. In Torsten's case, increase BIO_POOL_SIZE
> by one and check how it performs.

Sorry it seems simply increasing BIO_POOL_SIZE is not enough to fix
possible deadlocks. We need adding new mempool(s). Because when there
BIO_POOL_SIZE=2 and there are two concurrent reclaimers each take 1
reservation, they will deadlock each other when trying to take the
next bio at the raid1 level.

Thanks,
Fengguang
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