Re: [PATCH v3 0/1] mm: introduce put_user_page*(), placeholder versions

[Ira Weiny <ira.weiny@intel.com>]

On Wed, Mar 13, 2019 at 09:11:13AM +1100, Dave Chinner wrote:
> On Tue, Mar 12, 2019 at 03:39:33AM -0700, Ira Weiny wrote:
> > IMHO I don't think that the copy_file_range() is going to carry us through the
> > next wave of user performance requirements.  RDMA, while the first, is not the
> > only technology which is looking to have direct access to files.  XDP is
> > another.[1]
> 
> Sure, all I doing here was demonstrating that people have been
> trying to get local direct access to file mappings to DMA directly
> into them for a long time. Direct Io games like these are now
> largely unnecessary because we now have much better APIs to do
> zero-copy data transfer between files (which can do hardware offload
> if it is available!).
> 
> It's the long term pins that RDMA does that are the problem here.
> I'm asssuming that for XDP, you're talking about userspace zero copy
> from files to the network hardware and vice versa? transmit is
> simple (read-only mapping), but receive probably requires bpf
> programs to ensure that data (minus headers) in the incoming packet
> stream is correctly placed into the UMEM region?

Yes, exactly.

> 
> XDP receive seems pretty much like the same problem as RDMA writes
> into the file. i.e.  the incoming write DMAs are going to have to
> trigger page faults if the UMEM is a long term pin so the filesystem
> behaves correctly with this remote data placement.  I'd suggest that
> RDMA, XDP and anything other hardware that is going to pin
> file-backed mappings for the long term need to use the same "inform
> the fs of a write operation into it's mapping" mechanisms...

Yes agreed.  I have a hack patch I'm testing right now which allows the user to
take a LAYOUT lease from user space and GUP triggers on that, either allowing
or rejecting the pin based on the lease.  I think this is the first step of
what Jan suggested.[1]  There is a lot more detail to work out with what
happens if that lease needs to be broken.

> 
> And if we start talking about wanting to do peer-to-peer DMA from
> network/GPU device to storage device without going through a
> file-backed CPU mapping, we still need to have the filesystem
> involved to translate file offsets to storage locations the
> filesystem has allocated for the data and to lock them down for as
> long as the peer-to-peer DMA offload is in place.  In effect, this
> is the same problem as RDMA+FS-DAXs - the filesystem owns the file
> offset to storage location mapping and manages storage access
> arbitration, not the mm/vma mapping presented to userspace....

I've only daydreamed about Peer-to-peer transfers.  But yes I think this is the
direction we need to go.  But The details of doing a

GPU -> RDMA -> {network } -> RDMA -> FS DAX

And back again... without CPU/OS involvement are only a twinkle in my eye...
If that.

Ira

[1] https://lore.kernel.org/lkml/20190212160707.GA19076@quack2.suse.cz/