Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

[Christoph Hellwig]

On Sat, Jun 16, 2018 at 06:25:10PM -0700, john.hubbard@xxxxxxxxx wrote:
> From: John Hubbard <jhubbard@xxxxxxxxxx>
> 
> 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.