Subject: Re: [PATCH] user_events: Use alloc_pages instead of kzalloc for register pages

From: Beau Belgrave <[email-masked]>
Date: Fri, 11 Mar 2022 15:55:30 -0800

On Fri, Mar 11, 2022 at 06:44:40PM -0500, Steven Rostedt wrote:
> On Fri, 11 Mar 2022 18:33:43 -0500
> Steven Rostedt <[email-masked]> wrote:
> 
> > I tried it slightly differently. Why waist bits if MAX_EVENTS is greater
> > than the order. That is, make MAX_EVENTS depend on the order, not the other
> > way around.
> 
> Here's a version that keeps the reserving part as well as some bug fixes (I
> didn't even compile the previous version ;-)
> 
> -- Steve
> 
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index 4febc1d6ae72..e10ad057e797 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -30,9 +30,10 @@
>  
>  /*
>   * Limits how many trace_event calls user processes can create:
> - * Must be multiple of PAGE_SIZE.
> + * Must be a power of two of PAGE_SIZE.
>   */
> -#define MAX_PAGES 1
> +#define MAX_PAGE_ORDER 0
> +#define MAX_PAGES (1 << MAX_PAGE_ORDER)
>  #define MAX_EVENTS (MAX_PAGES * PAGE_SIZE)
>  
>  /* Limit how long of an event name plus args within the subsystem. */
> @@ -1622,16 +1623,17 @@ static void set_page_reservations(bool set)
>  
>  static int __init trace_events_user_init(void)
>  {
> +	struct page *pages;
>  	int ret;
>  
>  	/* Zero all bits beside 0 (which is reserved for failures) */
>  	bitmap_zero(page_bitmap, MAX_EVENTS);
>  	set_bit(0, page_bitmap);
>  
> -	register_page_data = kzalloc(MAX_EVENTS, GFP_KERNEL);
> -
> -	if (!register_page_data)
> +	pages = alloc_pages(GFP_KERNEL | __GFP_ZERO, MAX_PAGE_ORDER);
> +	if (!pages)
>  		return -ENOMEM;
> +	register_page_data = page_address(pages);
>  
>  	set_page_reservations(true);
>  
> @@ -1640,7 +1642,7 @@ static int __init trace_events_user_init(void)
>  	if (ret) {
>  		pr_warn("user_events could not register with tracefs\n");
>  		set_page_reservations(false);
> -		kfree(register_page_data);
> +		__free_pages(pages, MAX_PAGE_ORDER);
>  		return ret;
>  	}
>  

This looks good to me, I agree having the max events aligning to page
order makes more sense going forward.

Thanks,
-Beau