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

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Message-ID: <20190208044302.GA20493@dastard>
Date:   Fri, 8 Feb 2019 15:43:02 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Doug Ledford <dledford@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        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>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On Thu, Feb 07, 2019 at 04:55:37PM +0000, Christopher Lameter wrote:
> One approach that may be a clean way to solve this:
> 3. Filesystems that allow bypass of the page cache (like XFS / DAX) will
>    provide the virtual mapping when the PIN is done and DO NO OPERATIONS
>    on the longterm pinned range until the long term pin is removed.

So, ummm, how do we do block allocation then, which is done on
demand during writes?

IOWs, this requires the application to set up the file in the
correct state for the filesystem to lock it down so somebody else
can write to it.  That means the file can't be sparse, it can't be
preallocated (i.e. can't contain unwritten extents), it must have zeroes
written to it's full size before being shared because otherwise it
exposes stale data to the remote client (secure sites are going to
love that!), they can't be extended, etc.

IOWs, once the file is prepped and leased out for RDMA, it becomes
an immutable for the purposes of local access.

Which, essentially we can already do. Prep the file, map it
read/write, mark it immutable, then pin it via the longterm gup
interface which can do the necessary checks.

Simple to implement, the reasons for errors trying to modify the
file are already documented and queriable, and it's hard for
applications to get wrong.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com