linux-kernel - Re: [PATCH v3 0/1] mm: introduce put_user

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Date:   Tue, 12 Mar 2019 08:23:16 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Christopher Lameter <cl@...ux.com>, john.hubbard@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Benvenuti <benve@...co.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerome Glisse <jglisse@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Tom Talpey <tom@...pey.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v3 0/1] mm: introduce put_user_page*(), placeholder
 versions

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/