Re: [PATCH v11 rdma-next 5/7] RDMA/qedr: Use the common mmap API

[Gal Pressman <galpress@amazon.com>]

On 21/10/2019 20:33, Jason Gunthorpe wrote:
> On Sun, Oct 20, 2019 at 10:19:34AM +0300, Gal Pressman wrote:
>> On 24/09/2019 12:31, Michal Kalderon wrote:
>>>> From: Pressman, Gal <galpress@amazon.com>
>>>> Sent: Tuesday, September 24, 2019 11:50 AM
>>>>
>>>>
>>>>> On 23 Sep 2019, at 18:22, Michal Kalderon <mkalderon@marvell.com>
>>>> wrote:
>>>>>
>>>>>
>>>>>>
>>>>>> From: linux-rdma-owner@vger.kernel.org <linux-rdma-
>>>>>> owner@vger.kernel.org> On Behalf Of Gal Pressman
>>>>>>
>>>>>>> On 19/09/2019 20:55, Jason Gunthorpe wrote:
>>>>>>> Huh. If you recall we did all this work with the XA and the free
>>>>>>> callback because you said qedr was mmaping BAR pages that had some
>>>>>>> HW lifetime associated with them, and the HW resource was not to be
>>>>>>> reallocated until all users were gone.
>>>>>>>
>>>>>>> I think it would be a better example of this API if you pulled the
>>>>>>>
>>>>>>>    dev->ops->rdma_remove_user(dev->rdma_ctx, ctx->dpi);
>>>>>>>
>>>>>>> Into qedr_mmap_free().
>>>>>>>
>>>>>>> Then the rdma_user_mmap_entry_remove() will call it naturally as it
>>>>>>> does entry_put() and if we are destroying the ucontext we already
>>>>>>> know the mmaps are destroyed.
>>>>>>>
>>>>>>> Maybe the same basic comment for EFA, not sure. Gal?
>>>>>>
>>>>>> That's what EFA already does in this series, no?
>>>>>> We no longer remove entries on dealloc_ucontext, only when the entry
>>>>>> is freed.
>>>>>
>>>>> Actually, I think most of the discussions you had on the topic were
>>>>> with Gal, but Some apply to qedr as well, however, for qedr, the only
>>>>> hw resource we allocate (bar) is on alloc_ucontext , therefore we were
>>>>> safe to free it on dealloc_ucontext as all mappings were already
>>>>> zapped. Making the mmap_free a bit redundant for qedr except for the
>>>> need to free the entry.
>>>>>
>>>>> For EFA, it seemed the only operation delayed was freeing memory - I
>>>>> didn't see hw resources being freed... Gal?
>>>>
>>>> What do you mean by hw resources being freed? The BAR mappings are
>>>> under the device’s control and are associated to the lifetime of the UAR.
>>> The bar offset you get is from the device -> don't you release it back to the device
>>> So it can be allocated to a different application ? 
>>> In efa_com_create_qp -> you get offsets from the device that you use for mapping
>>> The bar -> are these unique for every call ? are they released during destroy_qp ? 
>>> Before this patch series mmap_entries_remove_free only freed the DMA pages, but
>>> Following this thread, it seemed the initial intention was that only the hw resources would
>>> Be delayed as the DMA pages are ref counted anyway.  I didn't see any delay to returning
>>> The bar offsets to the device. Thanks.
>> The BAR pages are being "freed" back to the device once the UAR is freed.
>> These pages' lifetime is under the control of the device so there's nothing the
>> driver needs to do, just make sure no one else is using them.
> 
> What frees the UAR?
> 
> In the mlx drivers this was done during destruction of the ucontext,
> but with this new mmap stuff it could be moved to the mmap_free..

Dealloc UAR is currently being called during dealloc_ucontext.
The mmap_free callback is per entry, how can dealloc_uar be moved there?