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

[Jason Gunthorpe]

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

Jason