powerpc: gup_hugepte() support THP based tail recounting

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>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 23 insertions, 1 deletions

diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index 78b14abded65..a618ef01bfad 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;