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Date:   Mon, 11 Feb 2019 13:49:45 -0700
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Ira Weiny <ira.weiny@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>
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 11:58:47AM -0800, Dan Williams wrote:
> On Mon, Feb 11, 2019 at 10:40 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > 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.
> 
> ...but there is. The blocks you thought you freed, especially if the
> system was under -ENOSPC pressure, won't actually be free after the
> successful ftruncate().

They won't be free after something dirties the existing mmap either.

Blocks also won't be free if you unlink a file that is currently still
open.

This isn't really new behavior for a FS.

Jason