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Date:   Wed, 19 Dec 2018 09:56:28 -0700
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Jan Kara <jack@suse.cz>
Cc:     Dave Chinner <david@fromorbit.com>,
        Jerome Glisse <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Matthew Wilcox <willy@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        John Hubbard <john.hubbard@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>, tom@talpey.com,
        Al Viro <viro@zeniv.linux.org.uk>, benve@cisco.com,
        Christoph Hellwig <hch@infradead.org>,
        Christopher Lameter <cl@linux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
        Doug Ledford <dledford@redhat.com>,
        Michal Hocko <mhocko@kernel.org>, mike.marciniszyn@intel.com,
        rcampbell@nvidia.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Message-ID: <20181219165628.GB30553@ziepe.ca>
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On Wed, Dec 19, 2018 at 12:35:40PM +0100, Jan Kara wrote:
> On Wed 19-12-18 21:28:25, Dave Chinner wrote:
> > On Tue, Dec 18, 2018 at 08:03:29PM -0700, Jason Gunthorpe wrote:
> > > On Wed, Dec 19, 2018 at 10:42:54AM +1100, Dave Chinner wrote:
> > > 
> > > > Essentially, what we are talking about is how to handle broken
> > > > hardware. I say we should just brun it with napalm and thermite
> > > > (i.e. taint the kernel with "unsupportable hardware") and force
> > > > wait_for_stable_page() to trigger when there are GUP mappings if
> > > > the underlying storage doesn't already require it.
> > > 
> > > If you want to ban O_DIRECT/etc from writing to file backed pages,
> > > then just do it.
> > 
> > O_DIRECT IO *isn't the problem*.
> 
> That is not true. O_DIRECT IO is a problem. In some aspects it is
> easier than the problem with RDMA but currently O_DIRECT IO can
> crash your machine or corrupt data the same way RDMA can. Just the
> race window is much smaller. So we have to fix the generic GUP
> infrastructure to make O_DIRECT IO work. I agree that fixing RDMA
> will likely require even more work like revokable leases or what
> not.

This is what I've understood, talking to all the experts. Dave? Why do
you think O_DIRECT is actually OK?

I agree the duration issue with RDMA is different, but don't forget,
O_DIRECT goes out to the network too and has potentially very long
timeouts as well.

If O_DIRECT works fine then lets use the same approach in RDMA??

Jason