Re: [PATCH v2 2/4] mm, page_owner: record page owner for each subpage

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Aug 20, 2019 at 03:18:26PM +0200, Vlastimil Babka wrote:
> Currently, page owner info is only recorded for the first page of a high-order
> allocation, and copied to tail pages in the event of a split page. With the
> plan to keep previous owner info after freeing the page, it would be benefical
> to record page owner for each subpage upon allocation. This increases the
> overhead for high orders, but that should be acceptable for a debugging option.
> 
> The order stored for each subpage is the order of the whole allocation. This
> makes it possible to calculate the "head" pfn and to recognize "tail" pages
> (quoted because not all high-order allocations are compound pages with true
> head and tail pages). When reading the page_owner debugfs file, keep skipping
> the "tail" pages so that stats gathered by existing scripts don't get inflated.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/page_owner.c | 40 ++++++++++++++++++++++++++++------------
>  1 file changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index addcbb2ae4e4..813fcb70547b 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -154,18 +154,23 @@ static noinline depot_stack_handle_t save_stack(gfp_t flags)
>  	return handle;
>  }
>  
> -static inline void __set_page_owner_handle(struct page_ext *page_ext,
> -	depot_stack_handle_t handle, unsigned int order, gfp_t gfp_mask)
> +static inline void __set_page_owner_handle(struct page *page,
> +	struct page_ext *page_ext, depot_stack_handle_t handle,
> +	unsigned int order, gfp_t gfp_mask)
>  {
>  	struct page_owner *page_owner;
> +	int i;
>  
> -	page_owner = get_page_owner(page_ext);
> -	page_owner->handle = handle;
> -	page_owner->order = order;
> -	page_owner->gfp_mask = gfp_mask;
> -	page_owner->last_migrate_reason = -1;
> +	for (i = 0; i < (1 << order); i++) {
> +		page_owner = get_page_owner(page_ext);
> +		page_owner->handle = handle;
> +		page_owner->order = order;
> +		page_owner->gfp_mask = gfp_mask;
> +		page_owner->last_migrate_reason = -1;
> +		__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
>  
> -	__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
> +		page_ext = lookup_page_ext(page + i);

Isn't it off-by-one? You are calculating page_ext for the next page,
right? And cant we just do page_ext++ here instead?

> +	}
>  }
>  
>  noinline void __set_page_owner(struct page *page, unsigned int order,
> @@ -178,7 +183,7 @@ noinline void __set_page_owner(struct page *page, unsigned int order,
>  		return;
>  
>  	handle = save_stack(gfp_mask);
> -	__set_page_owner_handle(page_ext, handle, order, gfp_mask);
> +	__set_page_owner_handle(page, page_ext, handle, order, gfp_mask);
>  }
>  
>  void __set_page_owner_migrate_reason(struct page *page, int reason)
> @@ -204,8 +209,11 @@ void __split_page_owner(struct page *page, unsigned int order)
>  
>  	page_owner = get_page_owner(page_ext);
>  	page_owner->order = 0;
> -	for (i = 1; i < (1 << order); i++)
> -		__copy_page_owner(page, page + i);
> +	for (i = 1; i < (1 << order); i++) {
> +		page_ext = lookup_page_ext(page + i);

Again, page_ext++?

> +		page_owner = get_page_owner(page_ext);
> +		page_owner->order = 0;
> +	}
>  }
>  
>  void __copy_page_owner(struct page *oldpage, struct page *newpage)
> @@ -483,6 +491,13 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  
>  		page_owner = get_page_owner(page_ext);
>  
> +		/*
> +		 * Don't print "tail" pages of high-order allocations as that
> +		 * would inflate the stats.
> +		 */
> +		if (!IS_ALIGNED(pfn, 1 << page_owner->order))
> +			continue;
> +
>  		/*
>  		 * Access to page_ext->handle isn't synchronous so we should
>  		 * be careful to access it.
> @@ -562,7 +577,8 @@ static void init_pages_in_zone(pg_data_t *pgdat, struct zone *zone)
>  				continue;
>  
>  			/* Found early allocated page */
> -			__set_page_owner_handle(page_ext, early_handle, 0, 0);
> +			__set_page_owner_handle(page, page_ext, early_handle,
> +						0, 0);
>  			count++;
>  		}
>  		cond_resched();
> -- 
> 2.22.0
> 
> 

-- 
 Kirill A. Shutemov