linux-kernel - Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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Date:   Wed, 19 Apr 2017 11:30:06 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Jerome Glisse <jglisse@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-rdma@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Steve Wise <swise@...ngridcomputing.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        Max Gurtovoy <maxg@...lanox.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Wed, Apr 19, 2017 at 11:19 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
> On 19/04/17 12:11 PM, Logan Gunthorpe wrote:
>>
>>
>> On 19/04/17 11:41 AM, Dan Williams wrote:
>>> No, not quite ;-). I still don't think we should require the non-HMM
>>> to pass NULL for all the HMM arguments. What I like about Logan's
>>> proposal is to have a separate create and register steps dev_pagemap.
>>> That way call paths that don't care about HMM specifics can just turn
>>> around and register the vanilla dev_pagemap.
>>
>> Would you necessarily even need a create step? I was thinking more along
>> the lines that struct dev_pagemap _could_ just be a member in another
>> structure. The caller would set the attributes they needed and pass it
>> to devm_memremap. (Similar to how we commonly do things with struct
>> device, et al). Potentially, that could also get rid of the need for the
>> *data pointer HMM is using to get back the struct hmm_devmem seeing
>> container_of could be used instead.
>
> Also, now that I've thought about it a little more, it _may_ be that
> many or all of the hmm specific fields in dev_pagemap could move to a
> containing struct too...

Yes, that's already how we handle struct page_map, it's an internal
implementation detail of devm_memremap_pages().

Letting others users do the container_of() arrangement means that
struct page_map needs to become public and move into struct
dev_pagemap directly.

...I think that encapsulation loss is worth it for the gain of clearly
separating the HMM-case from the base case.