Re: [PATCH] mm: add stackdepot information on page->private for tracking

[Vlastimil Babka]

On 10/6/22 05:19, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> 
> Private is vacant for most of Non-LRU pages while the user has explicitly
> operation on page->private via set_page_private, I would like introduce
> stackdepot information on page->private for a simplified tracking mechanism
> which could be help for kernel driver's memory leak.
> 
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>

This duplicates the existing page_owner functionality in a way that
unconditionally adds overhead to all kernels that have CONFIG_STACKDEPOT
enabled build-time (and also misses the need to initialize stackdepot properly).

Also wouldn't be suprised if some existing page->private users were actually
confused by the field suddenly being non-zero without their own action.

> ---
>  mm/page_alloc.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e5486d4..b79a503 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -75,6 +75,7 @@
>  #include <linux/khugepaged.h>
>  #include <linux/buffer_head.h>
>  #include <linux/delayacct.h>
> +#include <linux/stackdepot.h>
>  #include <asm/sections.h>
>  #include <asm/tlbflush.h>
>  #include <asm/div64.h>
> @@ -2464,6 +2465,25 @@ static inline bool should_skip_init(gfp_t flags)
>  	return (flags & __GFP_SKIP_ZERO);
>  }
>  
> +#ifdef CONFIG_STACKDEPOT
> +static noinline depot_stack_handle_t set_track_prepare(void)
> +{
> +       depot_stack_handle_t trace_handle;
> +       unsigned long entries[16];
> +       unsigned int nr_entries;
> +
> +       nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 3);
> +       trace_handle = stack_depot_save(entries, nr_entries, GFP_NOWAIT);
> +
> +       return trace_handle;
> +}
> +#else
> +static inline depot_stack_handle_t set_track_prepare(void)
> +{
> +       return 0;
> +}
> +#endif
> +
>  inline void post_alloc_hook(struct page *page, unsigned int order,
>  				gfp_t gfp_flags)
>  {
> @@ -2471,8 +2491,14 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>  			!should_skip_init(gfp_flags);
>  	bool init_tags = init && (gfp_flags & __GFP_ZEROTAGS);
>  	int i;
> +	depot_stack_handle_t stack_handle = set_track_prepare();
>  
> -	set_page_private(page, 0);
> +	/*
> +	 * Don't worry, user will cover private directly without checking
> +	 * this field and has ability to trace the page. This also will not
> +	 * affect expected state when freeing
> +	 */
> +	set_page_private(page, stack_handle);
>  	set_page_refcounted(page);
>  
>  	arch_alloc_page(page, order);