Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Michal Hocko]

On Fri 22-09-17 15:13:56, Reza Arbab wrote:
> The move_pages() syscall can be used to find the numa node where a page
> currently resides. This is not working for device public memory pages,
> which erroneously report -EFAULT (unmapped or zero page).
> 
> Enable by adding a FOLL_DEVICE flag for follow_page(), which
> move_pages() will use. This could be done unconditionally, but adding a
> flag seems like a safer change.

I do not understand purpose of this patch. What is the numa node of a
device memory?

> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Signed-off-by: Reza Arbab <arbab@xxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h | 1 +
>  mm/gup.c           | 2 +-
>  mm/migrate.c       | 2 +-
>  3 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f8c10d3..783cb57 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2368,6 +2368,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
>  #define FOLL_MLOCK	0x1000	/* lock present pages */
>  #define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
>  #define FOLL_COW	0x4000	/* internal GUP flag */
> +#define FOLL_DEVICE	0x8000	/* return device pages */
>  
>  static inline int vm_fault_to_errno(int vm_fault, int foll_flags)
>  {
> diff --git a/mm/gup.c b/mm/gup.c
> index b2b4d42..6fbad70 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -110,7 +110,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  		return NULL;
>  	}
>  
> -	page = vm_normal_page(vma, address, pte);
> +	page = _vm_normal_page(vma, address, pte, flags & FOLL_DEVICE);
>  	if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
>  		/*
>  		 * Only return device mapping pages in the FOLL_GET case since
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6954c14..dea0ceb 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1690,7 +1690,7 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
>  			goto set_status;
>  
>  		/* FOLL_DUMP to ignore special (like zero) pages */
> -		page = follow_page(vma, addr, FOLL_DUMP);
> +		page = follow_page(vma, addr, FOLL_DUMP | FOLL_DEVICE);
>  
>  		err = PTR_ERR(page);
>  		if (IS_ERR(page))
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs