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

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Message-ID: <20190215233828.GB30818@iweiny-DESK2.sc.intel.com>
Date:   Fri, 15 Feb 2019 15:38:29 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Doug Ledford <dledford@...hat.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, Feb 15, 2019 at 03:00:31PM -0700, Jason Gunthorpe wrote:
> On Fri, Feb 15, 2019 at 06:31:36PM +0000, Christopher Lameter wrote:
> > 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.
> 
> It is quite different, O_DIRECT modifies the physical blocks on the
> storage, bypassing the memory copy.
>

Really?  I thought O_DIRECT allowed the block drivers to write to/from user
space buffers.  But the _storage_ was still under the control of the block
drivers?

>
> RDMA modifies the memory copy.
> 
> pages are necessary to do RDMA, and those pages have to be flushed to
> disk.. So I'm not seeing how it can be disconnected from the page
> cache?

I don't disagree with this.

Ira

> 
> Jason