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

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Message-ID: <b3a0702baa3477748c3c243fa9d1c94f7b1954d6.camel@redhat.com>
Date:   Wed, 06 Feb 2019 16:12:06 -0500
From:   Doug Ledford <dledford@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>, 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, 2019-02-06 at 13:04 -0800, Dan Williams wrote:
> On Wed, Feb 6, 2019 at 12:14 PM Doug Ledford <dledford@...hat.com> wrote:
> > On Wed, 2019-02-06 at 11:45 -0800, Dan Williams wrote:
> > > 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?
> > > 
> > > If the
> > > RDMA application doesn't want it to happen, arrange for it by
> > > permissions or other coordination to prevent truncation,
> > 
> > I just argued the *exact* same thing, except from the other side: if you
> > want a guaranteed ability to truncate, then arrange the perms so the
> > RDMA or DAX capable things can't use the file.
> 
> That doesn't make sense. All we have to work with is rwx bits. It's
> possible to prevents writes / truncates. There's no permission bit for
> mmap, O_DIRECT and RDMA mappings, hence leases.

There's ownership.  What you can't open, you can't mmap or O_DIRECT or
whatever...

Regardless though, this is mostly moot as Dave's email makes it clear
the underlying issue that is the problem is not ftruncate, but other
things.
> > 

-- 
Doug Ledford <dledford@...hat.com>
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