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

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Date:   Wed, 13 Mar 2019 09:11:13 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Ira Weiny <ira.weiny@...el.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 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?

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...

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....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com