From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA
Date: Mon, 11 Feb 2019 11:06:54 -0700
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To: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>, Dave Chinner <david@fromorbit.com>, Christopher Lameter <cl@linux.com>, Doug Ledford <dledford@redhat.com>, Matthew Wilcox <willy@infradead.org>, Ira Weiny <ira.weiny@intel.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 09:22:58AM -0800, Dan Williams wrote:

> I honestly don't like the idea that random subsystems can pin down
> file blocks as a side effect of gup on the result of mmap. Recall that
> it's not just RDMA that wants this guarantee. It seems safer to have
> the file be in an explicit block-allocation-immutable-mode so that the
> fallocate man page can describe this error case. Otherwise how would
> you describe the scenarios under which FALLOC_FL_PUNCH_HOLE fails?

I rather liked CL's version of this - ftruncate/etc is simply racing
with a parallel pwrite - and it doesn't fail.

But it also doesnt' trucate/create a hole. Another thread wrote to it
right away and the 'hole' was essentially instantly reallocated. This
is an inherent, pre-existing, race in the ftrucate/etc APIs.

Jason