On Mon, Sep 10, 2012 at 04:02:39PM +0200, Eric Dumazet wrote:

On Mon, 2012-09-10 at 16:13 +0300, Kirill A. Shutemov wrote:

quoted

From: "Kirill A. Shutemov" <redacted>

H. Peter Anvin doesn't like huge zero page which sticks in memory forever
after the first allocation. Here's implementation of lockless refcounting
for huge zero page.

...

quoted

+static unsigned long get_huge_zero_page(void)
+{
+	struct page *zero_page;
+retry:
+	if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
+		return ACCESS_ONCE(huge_zero_pfn);
+
+	zero_page = alloc_pages(GFP_TRANSHUGE | __GFP_ZERO, HPAGE_PMD_ORDER);
+	if (!zero_page)
+		return 0;
+	if (cmpxchg(&huge_zero_pfn, 0, page_to_pfn(zero_page))) {
+		__free_page(zero_page);
+		goto retry;
+	}

This might break if preemption can happen here ?

The second thread might loop forever because huge_zero_refcount is 0,
and huge_zero_pfn not zero.

I fail to see why the second thread might loop forever. Long time yes, but
forever?

Yes, disabling preemption before alloc_pages() and enabling after
atomic_set() looks reasonable. Thanks.

If preemption already disabled, a comment would be nice.

quoted

+
+	/* We take additional reference here. It will be put back by shinker */

typo : shrinker

Thx.

quoted

+	atomic_set(&huge_zero_refcount, 2);
+	return ACCESS_ONCE(huge_zero_pfn);
+}
+

-- 
 Kirill A. Shutemov