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.
4.6.0-rc2 4.6.0-rc2
initonce-v1r20 multcheck-v1r20
Min alloc-odr0-1 359.00 ( 0.00%) 348.00 ( 3.06%)
Min alloc-odr0-2 260.00 ( 0.00%) 254.00 ( 2.31%)
Min alloc-odr0-4 214.00 ( 0.00%) 213.00 ( 0.47%)
Min alloc-odr0-8 186.00 ( 0.00%) 186.00 ( 0.00%)
Min alloc-odr0-16 173.00 ( 0.00%) 173.00 ( 0.00%)
Min alloc-odr0-32 165.00 ( 0.00%) 166.00 ( -0.61%)
Min alloc-odr0-64 162.00 ( 0.00%) 162.00 ( 0.00%)
Min alloc-odr0-128 161.00 ( 0.00%) 160.00 ( 0.62%)
Min alloc-odr0-256 170.00 ( 0.00%) 169.00 ( 0.59%)
Min alloc-odr0-512 181.00 ( 0.00%) 180.00 ( 0.55%)
Min alloc-odr0-1024 190.00 ( 0.00%) 188.00 ( 1.05%)
Min alloc-odr0-2048 196.00 ( 0.00%) 194.00 ( 1.02%)
Min alloc-odr0-4096 202.00 ( 0.00%) 199.00 ( 1.49%)
Min alloc-odr0-8192 205.00 ( 0.00%) 202.00 ( 1.46%)
Min alloc-odr0-16384 205.00 ( 0.00%) 203.00 ( 0.98%)
Again, the benefit is marginal but avoiding excessive branches is
important. Ideally the paths would not have to check these conditions at
all but regrettably abandoning the tests would make use-after-free bugs
much harder to detect.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
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 bc754d32aed6..3a60579342a5 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;
}
/*
@@ -1492,9 +1518,14 @@ 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
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