linux-kernel - Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

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Date:   Mon, 11 Feb 2019 11:58:47 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Ira Weiny <ira.weiny@...el.com>,
        Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Christopher Lameter <cl@...ux.com>,
        Doug Ledford <dledford@...hat.com>,
        lsf-pc@...ts.linux-foundation.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On Mon, Feb 11, 2019 at 10:40 AM Matthew Wilcox <willy@...radead.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().