Re: [PATCH v2 03/11] mm/gup: Applies counting method to monitor gup_pgd_range

[John Hubbard <jhubbard@nvidia.com>]

On 9/20/19 12:50 PM, Leonardo Bras wrote:
> As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
> monitor against THP split/collapse with the couting method, it's necessary
> to bound it with {start,end}_lockless_pgtbl_walk.
> 
> There are dummy functions, so it is not going to add any overhead on archs
> that don't use this method.
> 
> Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> ---
>  mm/gup.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 98f13ab37bac..675e4be27082 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2404,6 +2404,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>  			unsigned int gup_flags, struct page **pages)
>  {
>  	unsigned long addr, len, end;
> +	struct mm_struct *mm;
>  	int nr = 0, ret = 0;
>  
>  	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
> @@ -2421,9 +2422,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>  
>  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
>  	    gup_fast_permitted(start, end)) {
> +		mm = current->mm;
> +		start_lockless_pgtbl_walk(mm);
>  		local_irq_disable();

I'd also like a second opinion from the "core" -mm maintainers, but it seems like
there is now too much code around the gup_pgd_range() call. Especially since there
are two places where it's called--did you forget the other one in 
__get_user_pages_fast(), btw??

Maybe the irq handling and atomic counting should be moved into start/finish
calls, like this:

     start_gup_fast_walk()
     gup_pgd_range()
     finish_gup_fast_walk()

>  		gup_pgd_range(addr, end, gup_flags, pages, &nr);
>  		local_irq_enable();
> +		end_lockless_pgtbl_walk(mm);
>  		ret = nr;
>  	}
>  
> 



thanks,
-- 
John Hubbard
NVIDIA