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

[Andrew Morton <akpm@linux-foundation.org>]

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.

--- 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;
_