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

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Date:   Wed, 6 Feb 2019 11:45:56 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Doug Ledford <dledford@...hat.com>, Jan Kara <jack@...e.cz>,
        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>,
        Dave Chinner <david@...morbit.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On Wed, Feb 6, 2019 at 10:52 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Wed, Feb 06, 2019 at 10:35:04AM -0800, Matthew Wilcox wrote:
>
> > > Admittedly, I'm coming in late to this conversation, but did I miss the
> > > portion where that alternative was ruled out?
> >
> > That's my preferred option too, but the preponderance of opinion leans
> > towards "We can't give people a way to make files un-truncatable".
>
> I haven't heard an explanation why blocking ftruncate is worse than
> giving people a way to break RDMA using process by calling ftruncate??
>
> Isn't it exactly the same argument the other way?

No, I don't think it is. The lease is there to set the expectation of
getting out of the way, it's not a silent un-coordinated failure. The
user asked for it, the kernel is just honoring a valid request. If the
RDMA application doesn't want it to happen, arrange for it by
permissions or other coordination to prevent truncation, but once the
two conflicting / valid requests have arrived at the filesystem try to
move the result forward to the user requested state not block and fail
indefinitely.