Re: [PATCH 1/1] mm/page_ext: remove unnecessary stack_trace field

[Sergei Rogachev <rogachevsergei@gmail.com>]

On Tue, 27 Jan 2015 14:43:19 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Tue, 27 Jan 2015 00:52:12 +0300 Sergei Rogachev <rogachevsergei@gmail.com> wrote:
> 
> > Page owner uses the page_ext structure to keep meta-information
> > for every page in the system. The structure also contains a field
> > of type 'struct stack_trace', page owner uses this field during
> > invocation of the function save_stack_trace. It is easy to notice
> > that keeping a copy of this structure for every page in the system
> > is very inefficiently in terms of memory.
> > 
> > The patch removes this unnecessary field of page_ext and forces
> > page owner to use a stack_trace structure allocated on the stack.
> > 
> 
> Fair enough.
> 
> What do you think of the below tweaks?  Seems a bit neater, and it will
> cause all unmentioned fields to be zeroed out.  It's not a bug to leave
> .max_entries uninitialised in print_page_owner(), but it's nice not to
> leave loose ends.

Yes, with your fix the code looks much better!

> 
> --- a/mm/page_owner.c~mm-page_ext-remove-unnecessary-stack_trace-field-fix
> +++ a/mm/page_owner.c
> @@ -59,15 +59,14 @@ void __reset_page_owner(struct page *pag
>  
>  void __set_page_owner(struct page *page, unsigned int order, gfp_t gfp_mask)
>  {
> -	struct page_ext *page_ext;
> -	struct stack_trace trace;
> +	struct page_ext *page_ext = lookup_page_ext(page);
> +	struct stack_trace trace = {
> +		.nr_entries = 0,
> +		.max_entries = ARRAY_SIZE(page_ext->trace_entries),
> +		.entries = &page_ext->trace_entries[0],
> +		.skip = 3,
> +	};
>  
> -	page_ext = lookup_page_ext(page);
> -
> -	trace.nr_entries = 0;
> -	trace.max_entries = ARRAY_SIZE(page_ext->trace_entries);
> -	trace.entries = &page_ext->trace_entries[0];
> -	trace.skip = 3;
>  	save_stack_trace(&trace);
>  
>  	page_ext->order = order;
> @@ -83,8 +82,11 @@ print_page_owner(char __user *buf, size_
>  {
>  	int ret;
>  	int pageblock_mt, page_mt;
> -	struct stack_trace trace;
>  	char *kbuf;
> +	struct stack_trace trace = {
> +		.nr_entries = page_ext->nr_entries,
> +		.entries = &page_ext->trace_entries[0],
> +	};
>  
>  	kbuf = kmalloc(count, GFP_KERNEL);
>  	if (!kbuf)
> @@ -122,9 +124,6 @@ print_page_owner(char __user *buf, size_
>  	if (ret >= count)
>  		goto err;
>  
> -	trace.nr_entries = page_ext->nr_entries;
> -	trace.entries = &page_ext->trace_entries[0];
> -
>  	ret += snprint_stack_trace(kbuf + ret, count - ret, &trace, 0);
>  	if (ret >= count)
>  		goto err;
> _
>