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

[Leon Romanovsky <leon@kernel.org>]

On Fri, Dec 20, 2019 at 03:54:55PM -0800, John Hubbard wrote:
> On 12/20/19 10:29 AM, Leon Romanovsky wrote:
> ...
> >> $ ./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.
> >
>
> OK, I'm going to proceed for now on the assumption that I've got an overflow
> problem that happens when huge pages are pinned. If I can get more information,
> great, otherwise it's probably enough.
>
> One thing: for your repro, if you know the huge page size, and the system
> page size for that case, that would really help. Also the number of pins per
> page, more or less, that you'd expect. Because Jason says that only 2M huge
> pages are used...
>
> Because the other possibility is that the refcount really is going negative,
> likely due to a mismatched pin/unpin somehow.
>
> If there's not an obvious repro case available, but you do have one (is it easy
> to repro, though?), then *if* you have the time, I could point you to a github
> branch that reduces GUP_PIN_COUNTING_BIAS by, say, 4x, by applying this:

I'll see what I can do this Sunday.

>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bb44c4d2ada7..8526fd03b978 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1077,7 +1077,7 @@ static inline void put_page(struct page *page)
>   * get_user_pages and page_mkclean and other calls that race to set up page
>   * table entries.
>   */
> -#define GUP_PIN_COUNTING_BIAS (1U << 10)
> +#define GUP_PIN_COUNTING_BIAS (1U << 8)
>
>  void unpin_user_page(struct page *page);
>  void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
>
> If that fails to repro, then we would be zeroing in on the root cause.
>
> The branch is here (I just tested it and it seems healthy):
>
> git@github.com:johnhubbard/linux.git  pin_user_pages_tracking_v11_with_diags
>
>
>
> thanks,
> --
> John Hubbard
> NVIDIA