Re: [PATCH 10/10] IB: remove the unused usecnt field from struct ib_mr

Haggai Eran
2015-12-20T11:33:55Z

On 20/12/2015 11:31, Sagi Grimberg wrote:
>
>>> diff --git a/include/rdma/ib_verbs.h b/include/rdma/ib_verbs.h
>>> index 284916d..e45776e 100644
>>> --- a/include/rdma/ib_verbs.h
>>> +++ b/include/rdma/ib_verbs.h
>>> @@ -1306,7 +1306,6 @@ struct ib_mr {
>>> u64 iova;
>>> u32 length;
>>> unsigned int page_size;
>>> - atomic_t usecnt; /* count number of MWs */
>>> };
>>
>> This comment is part of Roland's uverbs commit.
>>
>> I wonder if LL driver supporting the IB_WR_BIND_MW op
>> ref the MR on port send and deref it on completion?
>
> I don't see why any driver would ref the underlying MR at
> post_send(IB_WR_BIND_MW) time. It doesn't really make sense.
>
> However, I think that the patch from Shani 6b52a12bc3fc ("IB/uverbs:
> Implement memory windows support in uverbs") is wrong. A memory
> window allocation should really reference the MR and not the PD (which
> is referenced by the MR). Otherwise the MR deregistration is allowed
> with windows bound to it. If this is the case then this field is not
> useless and we should fix ib_uverbs_alloc_mw()?
>
> Haggai, can you shed some light here? It's Shani's and your code.
A memory window takes reference on its PD during creation. It can only
be used by that PD, and it isn't bound to any memory region yet. You
are right that we want to prevent deregistration of an MR when there
are memory windows bound to it. This is suggested (although not
required) by the IBA specifications (section 10.6.7.2.6 Deregistering
Regions with Bound Windows). The problem with the usecnt field in ib_mr
as you wrote is that it needs to reflect what happens at bind /
invalidate time, but bind can happen in user-space without the kernel
knowing, and invalidation can similarly occur due to a
send-with-invalidate message being received. This is why in the
ConnectX-3 implementation of memory windows we didn't use the
ib_mr.usecnt field, but relied on a hardware implementation of the
reference count.

As I said, the spec also allows a device to proceed with deregistering
an MR, leaving an orphan memory window. I don't think we can prevent
this in the kernel, and I'm not sure it would be a good idea anyway, so
I think it is okay to remove the use_cnt field.

Regards,
Haggai