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>

diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index 78b14ab..a618ef0 100644 (file)
--- 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;