[PATCH 20/22] mm, page_alloc: Check multiple page fields with a single branch

[Mel Gorman]

Every page allocated or freed is checked for sanity to avoid corruptions
that are difficult to detect later.  A bad page could be due to a number of
fields. Instead of using multiple branches, this patch combines multiple
fields into a single branch. A detailed check is only necessary if that
check fails.

Signed-off-by: Mel Gorman <mgor...@techsingularity.net>
---
 mm/page_alloc.c | 55 +++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 43 insertions(+), 12 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8f53cb3853fc..59d7c627b29d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -784,10 +784,42 @@ static inline void __free_one_page(struct page *page,
        zone->free_area[order].nr_free++;
 }
 
+/*
+ * A bad page could be due to a number of fields. Instead of multiple branches,
+ * try and check multiple fields with one check. The caller must do a detailed
+ * check if necessary.
+ */
+static inline bool page_expected_state(struct page *page,
+                                       unsigned long check_flags)
+{
+       if (unlikely(atomic_read(&page->_mapcount) != -1))
+               return false;
+
+       if (unlikely((unsigned long)page->mapping |
+                       page_ref_count(page) |
+#ifdef CONFIG_MEMCG
+                       (unsigned long)page->mem_cgroup |
+#endif
+                       (page->flags & check_flags)))
+               return false;
+
+       return true;
+}
+
 static inline int free_pages_check(struct page *page)
 {
-       const char *bad_reason = NULL;
-       unsigned long bad_flags = 0;
+       const char *bad_reason;
+       unsigned long bad_flags;
+
+       if (page_expected_state(page, PAGE_FLAGS_CHECK_AT_FREE)) {
+               page_cpupid_reset_last(page);
+               page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
+               return 0;
+       }
+
+       /* Something has gone sideways, find it */
+       bad_reason = NULL;
+       bad_flags = 0;
 
        if (unlikely(atomic_read(&page->_mapcount) != -1))
                bad_reason = "nonzero mapcount";
@@ -803,14 +835,8 @@ static inline int free_pages_check(struct page *page)
        if (unlikely(page->mem_cgroup))
                bad_reason = "page still charged to cgroup";
 #endif
-       if (unlikely(bad_reason)) {
-               bad_page(page, bad_reason, bad_flags);
-               return 1;
-       }
-       page_cpupid_reset_last(page);
-       if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
-               page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
-       return 0;
+       bad_page(page, bad_reason, bad_flags);
+       return 1;
 }
 
 /*
@@ -1491,9 +1517,14 @@ static inline void expand(struct zone *zone, struct page 
*page,
  */
 static inline int check_new_page(struct page *page)
 {
-       const char *bad_reason = NULL;
-       unsigned long bad_flags = 0;
+       const char *bad_reason;
+       unsigned long bad_flags;
+
+       if (page_expected_state(page, PAGE_FLAGS_CHECK_AT_PREP|__PG_HWPOISON))
+               return 0;
 
+       bad_reason = NULL;
+       bad_flags = 0;
        if (unlikely(atomic_read(&page->_mapcount) != -1))
                bad_reason = "nonzero mapcount";
        if (unlikely(page->mapping != NULL))
-- 
2.6.4