[PATCH 2/3] sparc: gup_pte_range() support THP based tail recounting

[Andrea Arcangeli <aarcange@redhat.com>]

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>
---
 arch/sparc/mm/gup.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/arch/sparc/mm/gup.c b/arch/sparc/mm/gup.c
index a986b5d..afcebac 100644
--- a/arch/sparc/mm/gup.c
+++ b/arch/sparc/mm/gup.c
@@ -12,6 +12,17 @@
 #include <linux/rwsem.h>
 #include <asm/pgtable.h>
 
+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);
+}
+
 /*
  * The performance critical leaf functions are made noinline otherwise gcc
  * inlines everything into a single function which results in too much
@@ -56,6 +67,8 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
 			put_page(head);
 			return 0;
 		}
+		if (head != page)
+			get_huge_page_tail(page);
 
 		pages[*nr] = page;
 		(*nr)++;

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