[PATCH v3] mm/page_ext: Do not allocate space for page_ext->flags if not needed

[Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>]

There is 8 byte page_ext->flags field allocated per page whenever
CONFIG_PAGE_EXTENSION is enabled. However, not every user of page_ext
uses flags. Therefore, check whether flags is needed at least by one
user and if so allocate space for it.

For example when page_table_check is enabled, on a machine with 128G
of memory before the fix:

[    2.244288] allocated 536870912 bytes of page_ext
after the fix:
[    2.160154] allocated 268435456 bytes of page_ext

Also, add a comment before page_ext_operations that describes the
field, and remove check if need() is set, as that is now a required
field.

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 include/linux/page_ext.h | 19 +++++++++++++++++++
 mm/page_ext.c            | 14 ++++++++++++--
 mm/page_owner.c          |  1 +
 mm/page_table_check.c    |  1 +
 4 files changed, 33 insertions(+), 2 deletions(-)

Changelog:
v2: Fixed field name in page_owner.c that caused build error.
v3: Added comment before page_ext_operations, removed check if
    need is null.

diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
index 22be4582faae..be0b465c7d78 100644
--- a/include/linux/page_ext.h
+++ b/include/linux/page_ext.h
@@ -7,15 +7,31 @@
 #include <linux/stackdepot.h>
 
 struct pglist_data;
+
+/*
+ * Each Page Extension client must define page_ext_operations in
+ * page_ext_ops array.
+ * offset  offset to the client's data within page_ext. Offset is returned to
+ *         the client by page_ext_init.
+ * size    the size of the client data within page_ext.
+ * need()  function that returns true if client requires page_ext
+ * init()  (optional) called to initialize client once page_exts are allocated.
+ * using_shared_ext_flags  true when client is using shared page_ext->flags
+ *         field.
+ */
 struct page_ext_operations {
 	size_t offset;
 	size_t size;
 	bool (*need)(void);
 	void (*init)(void);
+	bool using_shared_ext_flags;
 };
 
 #ifdef CONFIG_PAGE_EXTENSION
 
+/*
+ * The page_ext_flags users must set using_shared_ext_flags to true.
+ */
 enum page_ext_flags {
 	PAGE_EXT_OWNER,
 	PAGE_EXT_OWNER_ALLOCATED,
@@ -31,6 +47,9 @@ enum page_ext_flags {
  * page_ext helps us add more information about the page.
  * All page_ext are allocated at boot or memory hotplug event,
  * then the page_ext for pfn always exists.
+ * Note: that if none of the users of page_ext is setting
+ * using_shared_ext_flags to true, the field cannot be used,
+ * as it will be overlapped with another data.
  */
 struct page_ext {
 	unsigned long flags;
diff --git a/mm/page_ext.c b/mm/page_ext.c
index 4ee522fd381c..d14dbd44fa49 100644
--- a/mm/page_ext.c
+++ b/mm/page_ext.c
@@ -71,6 +71,7 @@ static bool need_page_idle(void)
 }
 static struct page_ext_operations page_idle_ops __initdata = {
 	.need = need_page_idle,
+	.using_shared_ext_flags = true,
 };
 #endif
 
@@ -86,7 +87,7 @@ static struct page_ext_operations *page_ext_ops[] __initdata = {
 #endif
 };
 
-unsigned long page_ext_size = sizeof(struct page_ext);
+unsigned long page_ext_size;
 
 static unsigned long total_usage;
 static struct page_ext *lookup_page_ext(const struct page *page);
@@ -106,7 +107,16 @@ static bool __init invoke_need_callbacks(void)
 	bool need = false;
 
 	for (i = 0; i < entries; i++) {
-		if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
+		if (page_ext_ops[i]->need()) {
+			if (page_ext_ops[i]->using_shared_ext_flags) {
+				page_ext_size = sizeof(struct page_ext);
+				break;
+			}
+		}
+	}
+
+	for (i = 0; i < entries; i++) {
+		if (page_ext_ops[i]->need()) {
 			page_ext_ops[i]->offset = page_ext_size;
 			page_ext_size += page_ext_ops[i]->size;
 			need = true;
diff --git a/mm/page_owner.c b/mm/page_owner.c
index 2d27f532df4c..1a62ab3fc00e 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -99,6 +99,7 @@ struct page_ext_operations page_owner_ops = {
 	.size = sizeof(struct page_owner),
 	.need = need_page_owner,
 	.init = init_page_owner,
+	.using_shared_ext_flags = true,
 };
 
 static inline struct page_owner *get_page_owner(struct page_ext *page_ext)
diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 93e633c1d587..74cfe89d5f53 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -45,6 +45,7 @@ struct page_ext_operations page_table_check_ops = {
 	.size = sizeof(struct page_table_check),
 	.need = need_page_table_check,
 	.init = init_page_table_check,
+	.using_shared_ext_flags = false,
 };
 
 static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
-- 
2.39.0.314.g84b9a713c41-goog