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 11:06:54 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Christopher Lameter <cl@...ux.com>,
        Doug Ledford <dledford@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Ira Weiny <ira.weiny@...el.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 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