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

[Dan Williams]

On Mon, Feb 11, 2019 at 12:49 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx> 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.

An mmap write after a fault due to a hole punch is free to trigger
SIGBUS if the subsequent page allocation fails. So no, I don't see
them as the same unless you're allowing for the holder of the MR to
receive a re-fault failure.