Re: Enabling peer to peer device transactions for PCIe devices

Dan Williams
2016-11-23T18:40:47Z

On Wed, Nov 23, 2016 at 9:27 AM, Bart Van Assche
<bart.vanassche@sandisk.com> wrote:
> On 11/23/2016 09:13 AM, Logan Gunthorpe wrote:
>>
>> IMO any memory that has been registered for a P2P transaction should be
>> locked from being evicted. So if there's a get_user_pages call it needs
>> to be pinned until the put_page. The main issue being with the RDMA
>> case: handling an eviction when a chunk of memory has been registered as
>> an MR would be very tricky. The MR may be relied upon by another host
>> and the kernel would have to inform user-space the MR was invalid then
>> user-space would have to tell the remote application.
>
>
> Hello Logan,
>
> Are you aware that the Linux kernel already supports ODP (On Demand Paging)?
> See also the output of git grep -nHi on.demand.paging. See also
> https://www.openfabrics.org/images/eventpresos/workshops2014/DevWorkshop/presos/Tuesday/pdf/04_ODP_update.pdf.
>

I don't think that was designed for the case where the backing memory
is a special/static physical address range rather than anonymous
"System RAM", right?

I think we should handle the graphics P2P concerns separately from the
general P2P-DMA case since the latter does not require the higher
order memory management facilities. Using ZONE_DEVICE/DAX mappings to
avoid changes to every driver that wants to support P2P-DMA separately
from typical DMA still seems the path of least resistance.