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

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Message-ID: <01000168f26d9e0c-aef3255a-5059-4657-b241-dae66663bbea-000000@email.amazonses.com>
Date:   Fri, 15 Feb 2019 18:31:36 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Dave Chinner <david@...morbit.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Doug Ledford <dledford@...hat.com>,
        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>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP
 usage by RDMA

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.