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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Mon, 11 Feb 2019 10:15:33 -0800
Message-ID: <CAPcyv4hdVwn0L8060_wxqDV0XBOwiUojSYjF+u+ugzLoQpcHzw@mail.gmail.com>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA
To:     Jason Gunthorpe <jgg@ziepe.ca>
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 10:07 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> 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.

If options are telling the truth with a potentially unexpected error,
or lying that operation succeeded when it will be immediately undone,
I'd choose the former.