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.
