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

Dan Williams
2019-12-21T00:32:32Z

On Fri, Dec 20, 2019 at 5:34 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Thu, Dec 19, 2019 at 01:13:54PM -0800, John Hubbard wrote:
> > On 12/19/19 1:07 PM, 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.
> >
> > There's already a limit, it's just a much larger one. :) What does "no limit"
> > really mean, numerically, to you in this case?
>
> I guess I mean 'hidden limit' - hitting the limit and failing would
> be managable.
>
> I think 7 is probably too low though, but we are not using 1GB huge
> pages, only 2M..

What about RDMA to 1GB-hugetlbfs and 1GB-device-dax mappings?