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

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Date:   Thu, 14 Feb 2019 12:50:49 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Christopher Lameter <cl@...ux.com>,
        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 Thu, Feb 14, 2019 at 03:26:22PM -0500, Jerome Glisse wrote:
> On Mon, Feb 11, 2019 at 11:06:54AM -0700, Jason Gunthorpe wrote:
> > But it also doesnt' trucate/create a hole. Another thread wrote to it
> > right away and the 'hole' was essentially instantly reallocated. This
> > is an inherent, pre-existing, race in the ftrucate/etc APIs.
> 
> So it is kind of a // point to this, but direct I/O do "truncate" pages
> or more exactly after a write direct I/O invalidate_inode_pages2_range()
> is call and it will try to unmap and remove from page cache all pages
> that have been written too.

Hang on.  Pages are tossed out of the page cache _before_ an O_DIRECT
write starts.  The only way what you're describing can happen is if
there's a race between an O_DIRECT writer and an mmap.  Which is either
an incredibly badly written application or someone trying an exploit.