Re: [PATCH v11 00/25] mm/gup: track dma-pinned pages: FOLL_PIN

[Leon Romanovsky <leon@kernel.org>]

On Thu, Dec 19, 2019 at 05:07:43PM -0400, Jason Gunthorpe wrote:
> On Thu, Dec 19, 2019 at 12:30:31PM -0800, John Hubbard wrote:
> > On 12/19/19 5:26 AM, Leon Romanovsky wrote:
> > > On Mon, Dec 16, 2019 at 02:25:12PM -0800, John Hubbard wrote:
> > > > Hi,
> > > >
> > > > This implements an API naming change (put_user_page*() -->
> > > > unpin_user_page*()), and also implements tracking of FOLL_PIN pages. It
> > > > extends that tracking to a few select subsystems. More subsystems will
> > > > be added in follow up work.
> > >
> > > Hi John,
> > >
> > > The patchset generates kernel panics in our IB testing. In our tests, we
> > > allocated single memory block and registered multiple MRs using the single
> > > block.
> > >
> > > The possible bad flow is:
> > >   ib_umem_geti() ->
> > >    pin_user_pages_fast(FOLL_WRITE) ->
> > >     internal_get_user_pages_fast(FOLL_WRITE) ->
> > >      gup_pgd_range() ->
> > >       gup_huge_pd() ->
> > >        gup_hugepte() ->
> > >         try_grab_compound_head() ->
> >
> > Hi Leon,
> >
> > Thanks very much for the detailed report! So we're overflowing...
> >
> > At first look, this seems likely to be hitting a weak point in the
> > GUP_PIN_COUNTING_BIAS-based design, one that I believed could be deferred
> > (there's a writeup in Documentation/core-api/pin_user_page.rst, lines
> > 99-121). Basically it's pretty easy to overflow the page->_refcount
> > with huge pages if the pages have a *lot* of subpages.
> >
> > We can only do about 7 pins on 1GB huge pages that use 4KB subpages.
>
> Considering that establishing these pins is entirely under user
> control, we can't have a limit here.
>
> If the number of allowed pins are exhausted then the
> pin_user_pages_fast() must fail back to the user.
>
> > 3. It would be nice if I could reproduce this. I have a two-node mlx5 Infiniband
> > test setup, but I have done only the tiniest bit of user space IB coding, so
> > if you have any test programs that aren't too hard to deal with that could
> > possibly hit this, or be tweaked to hit it, I'd be grateful. Keeping in mind
> > that I'm not an advanced IB programmer. At all. :)
>
> Clone this:
>
> https://github.com/linux-rdma/rdma-core.git
>
> Install all the required deps to build it (notably cython), see the README.md
>
> $ ./build.sh
> $ build/bin/run_tests.py
>
> If you get things that far I think Leon can get a reproduction for you

I'm not so optimistic about that.

Thanks

>
> Jason