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

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Dec 20, 2019 at 1:22 AM Jan Kara <jack@suse.cz> wrote:
>
> On Thu 19-12-19 12:30:31, 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.
> > Do you have any idea how many pins (repeated pins on the same page, which
> > it sounds like you have) might be involved in your test case,
> > and the huge page and system page sizes? That would allow calculating
> > if we're likely overflowing for that reason.
> >
> > So, ideas and next steps:
> >
> > 1. Assuming that you *are* hitting this, I think I may have to fall back to
> > implementing the "deferred" part of this design, as part of this series, after
> > all. That means:
> >
> >   For the pin/unpin calls at least, stop treating all pages as if they are
> >   a cluster of PAGE_SIZE pages; instead, retrieve a huge page as one page.
> >   That's not how it works now, and the need to hand back a huge array of
> >   subpages is part of the problem. This affects the callers too, so it's not
> >   a super quick change to make. (I was really hoping not to have to do this
> >   yet.)
>
> Does that mean that you would need to make all GUP users huge page aware?
> Otherwise I don't see how what you suggest would work... And I don't think
> making all GUP users huge page aware is realistic (effort-wise) or even
> wanted (maintenance overhead in all those places).
>
> I believe there might be also a different solution for this: For
> transparent huge pages, we could find a space in 'struct page' of the
> second page in the huge page for proper pin counter and just account pins
> there so we'd have full width of 32-bits for it.

That would require THP accounting for dax pages. It is something that
was probably going to be needed, but this would seem to force the
issue.