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authorJoonsoo Kim <iamjoonsoo.kim@lge.com>2015-02-10 14:09:35 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2015-02-10 14:30:30 -0800
commitccaafd7fd039aebc9359a9799f8558b01f1c2adc (patch)
treec4a32ede5bb661489da8846cfe947bcb251f6c11
parent9aabf810a67cd97e2d1a48f0bab338b7680f1929 (diff)
mm: don't use compound_head() in virt_to_head_page()
compound_head() is implemented with assumption that there would be race condition when checking tail flag. This assumption is only true when we try to access arbitrary positioned struct page. The situation that virt_to_head_page() is called is different case. We call virt_to_head_page() only in the range of allocated pages, so there is no race condition on tail flag. In this case, we don't need to handle race condition and we can reduce overhead slightly. This patch implements compound_head_fast() which is similar with compound_head() except tail flag race handling. And then, virt_to_head_page() uses this optimized function to improve performance. I saw 1.8% win in a fast-path loop over kmem_cache_alloc/free, (14.063 ns -> 13.810 ns) if target object is on tail page. Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com> Acked-by: Christoph Lameter <cl@linux.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: David Rientjes <rientjes@google.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: Jesper Dangaard Brouer <brouer@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/mm.h27
1 files changed, 26 insertions, 1 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd5ea3016fc4..2c6fd3c5424a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -446,6 +446,12 @@ static inline struct page *compound_head_by_tail(struct page *tail)
return tail;
}
+/*
+ * Since either compound page could be dismantled asynchronously in THP
+ * or we access asynchronously arbitrary positioned struct page, there
+ * would be tail flag race. To handle this race, we should call
+ * smp_rmb() before checking tail flag. compound_head_by_tail() did it.
+ */
static inline struct page *compound_head(struct page *page)
{
if (unlikely(PageTail(page)))
@@ -454,6 +460,18 @@ static inline struct page *compound_head(struct page *page)
}
/*
+ * If we access compound page synchronously such as access to
+ * allocated page, there is no need to handle tail flag race, so we can
+ * check tail flag directly without any synchronization primitive.
+ */
+static inline struct page *compound_head_fast(struct page *page)
+{
+ if (unlikely(PageTail(page)))
+ return page->first_page;
+ return page;
+}
+
+/*
* The atomic page->_mapcount, starts from -1: so that transitions
* both from it and to it can be tracked, using atomic_inc_and_test
* and atomic_add_negative(-1).
@@ -531,7 +549,14 @@ static inline void get_page(struct page *page)
static inline struct page *virt_to_head_page(const void *x)
{
struct page *page = virt_to_page(x);
- return compound_head(page);
+
+ /*
+ * We don't need to worry about synchronization of tail flag
+ * when we call virt_to_head_page() since it is only called for
+ * already allocated page and this page won't be freed until
+ * this virt_to_head_page() is finished. So use _fast variant.
+ */
+ return compound_head_fast(page);
}
/*