linux-kernel - Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user

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Date:   Mon, 18 Jun 2018 00:56:50 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     john.hubbard@...il.com
Cc:     Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

On Sat, Jun 16, 2018 at 06:25:10PM -0700, john.hubbard@...il.com wrote:
> From: John Hubbard <jhubbard@...dia.com>
> 
> This fixes a few problems that come up when using devices (NICs, GPUs,
> for example) that want to have direct access to a chunk of system (CPU)
> memory, so that they can DMA to/from that memory. Problems [1] come up
> if that memory is backed by persistence storage; for example, an ext4
> file system. I've been working on several customer bugs that are hitting
> this, and this patchset fixes those bugs.

What happens if we do get_user_page from two different threads or even
processes on the same page?  As far as I can tell from your patch
the first one finishing the page will clear the bit and then we are
back to no protection.

Note that you can reproduce such a condition trivially using direct
I/O reads or writes.