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From: Andrea Arcangeli <aarcange@redhat.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan.kim@gmail.com>,
        Michel Lespinasse <walken@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Hugh Dickins <hughd@google.com>,
        Johannes Weiner <jweiner@redhat.com>, Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@suse.de>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Shaohua Li <shaohua.li@intel.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: [PATCH 4/4] powerpc: gup_hugepte() support THP based tail recounting
Date: Sun, 16 Oct 2011 22:40:39 +0200
Message-Id: <1318797639-26962-5-git-send-email-aarcange@redhat.com>
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Up to this point the code assumed old refcounting for hugepages
(pre-thp). This updates the code directly to the thp mapcount tail page
refcounting.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index 78b14ab..a618ef0 100644
--- a/arch/powerpc/mm/hugetlbpage.c
+++ b/arch/powerpc/mm/hugetlbpage.c
@@ -385,12 +385,23 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 	return NULL;
 }
 
+static inline void get_huge_page_tail(struct page *page)
+{
+	/*
+	 * __split_huge_page_refcount() cannot run
+	 * from under us.
+	 */
+	VM_BUG_ON(page_mapcount(page) < 0);
+	VM_BUG_ON(atomic_read(&page->_count) != 0);
+	atomic_inc(&page->_mapcount);
+}
+
 static noinline int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 		       unsigned long end, int write, struct page **pages, int *nr)
 {
 	unsigned long mask;
 	unsigned long pte_end;
-	struct page *head, *page;
+	struct page *head, *page, *tail;
 	pte_t pte;
 	int refs;
 
@@ -413,6 +424,7 @@ static noinline int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long add
 	head = pte_page(pte);
 
 	page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
+	tail = page;
 	do {
 		VM_BUG_ON(compound_head(page) != head);
 		pages[*nr] = page;
@@ -431,6 +443,16 @@ static noinline int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long add
 		*nr -= refs;
 		while (refs--)
 			put_page(head);
+	} else {
+		/*
+		 * Any tail page need their mapcount reference taken
+		 * before we return.
+		 */
+		while (refs--) {
+			if (PageTail(tail))
+				get_huge_page_tail(tail);
+			tail++;
+		}
 	}
 
 	return 1;
-- 
1.7.3.4