mm/page_ext: support extra space allocation by page_ext user

Until now, if some page_ext users want to use it's own field on
page_ext, it should be defined in struct page_ext by hard-coding.  It
has a problem that wastes memory in following situation.

  struct page_ext {
   #ifdef CONFIG_A
  	int a;
   #endif
   #ifdef CONFIG_B
  	int b;
   #endif
  };

Assume that kernel is built with both CONFIG_A and CONFIG_B.  Even if we
enable feature A and doesn't enable feature B at runtime, each entry of
struct page_ext takes two int rather than one int.  It's undesirable
result so this patch tries to fix it.

To solve above problem, this patch implements to support extra space
allocation at runtime.  When need() callback returns true, it's extra
memory requirement is summed to entry size of page_ext.  Also, offset
for each user's extra memory space is returned.  With this offset, user
can use this extra space and there is no need to define needed field on
page_ext by hard-coding.

This patch only implements an infrastructure.  Following patch will use
it for page_owner which is only user having it's own fields on page_ext.

Link: http://lkml.kernel.org/r/1471315879-32294-6-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/page_ext.h

@@ -7,6 +7,8 @@
 
 struct pglist_data;
 struct page_ext_operations {
+	size_t offset;
+	size_t size;
 	bool (*need)(void);
 	void (*init)(void);
 };

mm/page_alloc.c

@@ -687,7 +687,7 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
 		__mod_zone_freepage_state(zone, (1 << order), migratetype);
 }
 #else
-struct page_ext_operations debug_guardpage_ops = { NULL, };
+struct page_ext_operations debug_guardpage_ops;
 static inline bool set_page_guard(struct zone *zone, struct page *page,
 			unsigned int order, int migratetype) { return false; }
 static inline void clear_page_guard(struct zone *zone, struct page *page,

mm/page_ext.c

@@ -42,6 +42,11 @@
  * and page extension core can skip to allocate memory. As result,
  * none of memory is wasted.
  *
+ * When need callback returns true, page_ext checks if there is a request for
+ * extra memory through size in struct page_ext_operations. If it is non-zero,
+ * extra space is allocated for each page_ext entry and offset is returned to
+ * user through offset in struct page_ext_operations.
+ *
  * The init callback is used to do proper initialization after page extension
  * is completely initialized. In sparse memory system, extra memory is
  * allocated some time later than memmap is allocated. In other words, lifetime
@@ -66,18 +71,24 @@ static struct page_ext_operations *page_ext_ops[] = {
 };
 
 static unsigned long total_usage;
+static unsigned long extra_mem;
 
 static bool __init invoke_need_callbacks(void)
 {
 	int i;
 	int entries = ARRAY_SIZE(page_ext_ops);
+	bool need = false;
 
 	for (i = 0; i < entries; i++) {
-		if (page_ext_ops[i]->need && page_ext_ops[i]->need())
-			return true;
+		if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
+			page_ext_ops[i]->offset = sizeof(struct page_ext) +
+						extra_mem;
+			extra_mem += page_ext_ops[i]->size;
+			need = true;
+		}
 	}
 
-	return false;
+	return need;
 }
 
 static void __init invoke_init_callbacks(void)
@@ -91,6 +102,16 @@ static void __init invoke_init_callbacks(void)
 	}
 }
 
+static unsigned long get_entry_size(void)
+{
+	return sizeof(struct page_ext) + extra_mem;
+}
+
+static inline struct page_ext *get_entry(void *base, unsigned long index)
+{
+	return base + get_entry_size() * index;
+}
+
 #if !defined(CONFIG_SPARSEMEM)
 
 
@@ -121,7 +142,7 @@ struct page_ext *lookup_page_ext(struct page *page)
 #endif
 	index = pfn - round_down(node_start_pfn(page_to_nid(page)),
 					MAX_ORDER_NR_PAGES);
-	return base + index;
+	return get_entry(base, index);
 }
 
 static int __init alloc_node_page_ext(int nid)
@@ -143,7 +164,7 @@ static int __init alloc_node_page_ext(int nid)
 		!IS_ALIGNED(node_end_pfn(nid), MAX_ORDER_NR_PAGES))
 		nr_pages += MAX_ORDER_NR_PAGES;
 
-	table_size = sizeof(struct page_ext) * nr_pages;
+	table_size = get_entry_size() * nr_pages;
 
 	base = memblock_virt_alloc_try_nid_nopanic(
 			table_size, PAGE_SIZE, __pa(MAX_DMA_ADDRESS),
@@ -196,7 +217,7 @@ struct page_ext *lookup_page_ext(struct page *page)
 	if (!section->page_ext)
 		return NULL;
 #endif
-	return section->page_ext + pfn;
+	return get_entry(section->page_ext, pfn);
 }
 
 static void *__meminit alloc_page_ext(size_t size, int nid)
@@ -229,7 +250,7 @@ static int __meminit init_section_page_ext(unsigned long pfn, int nid)
 	if (section->page_ext)
 		return 0;
 
-	table_size = sizeof(struct page_ext) * PAGES_PER_SECTION;
+	table_size = get_entry_size() * PAGES_PER_SECTION;
 	base = alloc_page_ext(table_size, nid);
 
 	/*
@@ -249,7 +270,7 @@ static int __meminit init_section_page_ext(unsigned long pfn, int nid)
 	 * we need to apply a mask.
 	 */
 	pfn &= PAGE_SECTION_MASK;
-	section->page_ext = base - pfn;
+	section->page_ext = (void *)base - get_entry_size() * pfn;
 	total_usage += table_size;
 	return 0;
 }
@@ -262,7 +283,7 @@ static void free_page_ext(void *addr)
 		struct page *page = virt_to_page(addr);
 		size_t table_size;
 
-		table_size = sizeof(struct page_ext) * PAGES_PER_SECTION;
+		table_size = get_entry_size() * PAGES_PER_SECTION;
 
 		BUG_ON(PageReserved(page));
 		free_pages_exact(addr, table_size);
@@ -277,7 +298,7 @@ static void __free_page_ext(unsigned long pfn)
 	ms = __pfn_to_section(pfn);
 	if (!ms || !ms->page_ext)
 		return;
-	base = ms->page_ext + pfn;
+	base = get_entry(ms->page_ext, pfn);
 	free_page_ext(base);
 	ms->page_ext = NULL;
 }