Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

[John Hubbard]

On 9/15/20 4:22 PM, Jason Gunthorpe wrote:
> On Tue, Sep 15, 2020 at 05:33:30PM -0400, Peter Xu wrote:
>
> > > RDMA doesn't ever use !WRITE
> > >
> > > I'm guessing #5 is the issue, just with a different ordering. If the
> > > #3 pin_user_pages() preceeds the #2 fork, don't we get to the same #5?
> >
> > Right, but only if without MADV_DONTFORK?
>
> Yes, results are that MADV_DONTFORK resolves the issue for initial
> tests. I should know if a bigger test suite passes in a few days.
>   
> > > > If this is a problem, we may still need the fix patch (maybe not as urgent as
> > > > before at least).  But I'd like to double confirm, just in case I miss some
> > > > obvious facts above.
> > >
> > > I'm worred that the sudden need to have MAD_DONTFORK is going to be a
> > > turn into a huge regression. It already blew up our first level of
> > > synthetic test cases. I'm worried what we will see when the
> > > application suite is run in a few months :\
> >
> > For my own preference I'll consider changing kernel behavior if the impact is
> > still under control (the performance report of 30%+ boost is also attractive
> > after the simplify-cow patch).  The other way is to maintain the old reuse
> > logic forever, that'll be another kind of burden.  Seems no easy way on either
> > side...
>
> It seems very strange that a physical page exclusively owned by a
> process can become copied if pin_user_pages() is active and the
> process did fork() at some point.
>
> Could the new pin_user_pages() logic help here? eg the
> GUP_PIN_COUNTING_BIAS stuff?
>
> Could the COW code consider a refcount of GUP_PIN_COUNTING_BIAS + 1 as
> being owned by the current mm and not needing COW? The DMA pin would
> be 'invisible' for COW purposes?


Please do be careful to use the API, rather than the implementation. The
FOLL_PIN refcounting system results in being able to get a "maybe
DMA-pinned", or a "definitely not DMA-pinned", via this API call:

    static inline bool page_maybe_dma_pinned(struct page *page)

...which does *not* always use GUP_PIN_COUNTING_BIAS to provide that
answer.

thanks,
--
John Hubbard
NVIDIA