From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA
Date: Mon, 11 Feb 2019 10:40:40 -0800
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To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Ira Weiny <ira.weiny@intel.com>, Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.cz>, Dave Chinner <david@fromorbit.com>, Christopher Lameter <cl@linux.com>, Doug Ledford <dledford@redhat.com>, lsf-pc@lists.linux-foundation.org, linux-rdma <linux-rdma@vger.kernel.org>, Linux MM <linux-mm@kvack.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, John Hubbard <jhubbard@nvidia.com>, Jerome Glisse <jglisse@redhat.com>, Michal Hocko <mhocko@kernel.org>
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On Mon, Feb 11, 2019 at 11:26:49AM -0700, Jason Gunthorpe wrote:
> On Mon, Feb 11, 2019 at 10:19:22AM -0800, Ira Weiny wrote:
> > What if user space then writes to the end of the file with a regular write?
> > Does that write end up at the point they truncated to or off the end of the
> > mmaped area (old length)?
> 
> IIRC it depends how the user does the write..
> 
> pwrite() with a given offset will write to that offset, re-extending
> the file if needed
> 
> A file opened with O_APPEND and a write done with write() should
> append to the new end
> 
> A normal file with a normal write should write to the FD's current
> seek pointer.
> 
> I'm not sure what happens if you write via mmap/msync.
> 
> RDMA is similar to pwrite() and mmap.

A pertinent point that you didn't mention is that ftruncate() does not change
the file offset.  So there's no user-visible change in behaviour.