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

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Date:   Wed, 6 Feb 2019 20:24:17 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Doug Ledford <dledford@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Jan Kara <jack@...e.cz>, Ira Weiny <ira.weiny@...el.com>,
        lsf-pc@...ts.linux-foundation.org, linux-rdma@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.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, 6 Feb 2019, Matthew Wilcox wrote:

> >
> > Coming in late here too but isnt the only DAX case that we are concerned
> > about where there was an mmap with the O_DAX option to do direct write
>
> There is no O_DAX option.  There's mount -o dax, but there's nothing that
> a program does to say "Use DAX".

Hmmm... I thought that a file handle must have a special open mode to
actually to a dax map. Looks like that is not the case.

> > though? If we only allow this use case then we may not have to worry about
> > long term GUP because DAX mapped files will stay in the physical location
> > regardless.
>
> ... except for truncate.  And now that I think about it, there was a
> desire to support hot-unplug which also needed revoke.

Well but that requires that the application unmaps the file.

> > Maybe we can solve the long term GUP problem through the requirement that
> > user space acquires some sort of means to pin the pages? In the DAX case
> > this is given by the filesystem and the hardware will basically take care
> > of writeback.
>
> It's not given by the filesystem.

DAX provides a mapping to physical persistent memory that
does not go away. Or its a block device.

>
> > In case of anonymous memory this can be guaranteed otherwise and is less
> > critical since these pages are not part of the pagecache and are not
> > subject to writeback.
>
> but are subject to being swapped out?

Well that is controlled by mlock and could also involve other means like
disabling swap.