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Date:   Mon, 11 Feb 2019 10:40:40 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Ira Weiny <ira.weiny@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
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        Christopher Lameter <cl@linux.com>,
        Doug Ledford <dledford@redhat.com>,
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        Linux MM <linux-mm@kvack.org>,
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        John Hubbard <jhubbard@nvidia.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Michal Hocko <mhocko@kernel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA
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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.