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Date:   Fri, 15 Feb 2019 18:31:36 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Matthew Wilcox <willy@infradead.org>
cc:     Dave Chinner <david@fromorbit.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dan Williams <dan.j.williams@intel.com>,
        Jan Kara <jack@suse.cz>, Doug Ledford <dledford@redhat.com>,
        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>,
        Michal Hocko <mhocko@kernel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP
 usage by RDMA
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On Fri, 15 Feb 2019, Matthew Wilcox wrote:

> > Since RDMA is something similar: Can we say that a file that is used for
> > RDMA should not use the page cache?
>
> That makes no sense.  The page cache is the standard synchronisation point
> for filesystems and processes.  The only problems come in for the things
> which bypass the page cache like O_DIRECT and DAX.

It makes a lot of sense since the filesystems play COW etc games with the
pages and RDMA is very much like O_DIRECT in that the pages are modified
directly under I/O. It also bypasses the page cache in case you have
not noticed yet.

Both filesysetms and RDMA acting on a page cache at
the same time lead to the mess that we are trying to solve.