thp: alter compound get_page/put_page

Alter compound get_page/put_page to keep references on subpages too, in
order to allow __split_huge_page_refcount to split an hugepage even while
subpages have been pinned by one of the get_user_pages() variants.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 24 insertions, 0 deletions

diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
index d7efdbf640c7..fec13200868f 100644
--- a/arch/powerpc/mm/gup.c
+++ b/arch/powerpc/mm/gup.c
@@ -16,6 +16,16 @@
 
 #ifdef __HAVE_ARCH_PTE_SPECIAL
 
+static inline void get_huge_page_tail(struct page *page)
+{
+        /*
+         * __split_huge_page_refcount() cannot run
+         * from under us.
+         */
+        VM_BUG_ON(atomic_read(&page->_count) < 0);
+        atomic_inc(&page->_count);
+}
+
 /*
  * The performance critical leaf functions are made noinline otherwise gcc
  * inlines everything into a single function which results in too much
@@ -47,6 +57,8 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
                         put_page(page);
                         return 0;
                 }
+                if (PageTail(page))
+                        get_huge_page_tail(page);
                 pages[*nr] = page;
                 (*nr)++;
 
diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 738e6593799d..06f56fcf9a77 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -105,6 +105,16 @@ static inline void get_head_page_multiple(struct page *page, int nr)
         atomic_add(nr, &page->_count);
 }
 
+static inline void get_huge_page_tail(struct page *page)
+{
+        /*
+         * __split_huge_page_refcount() cannot run
+         * from under us.
+         */
+        VM_BUG_ON(atomic_read(&page->_count) < 0);
+        atomic_inc(&page->_count);
+}
+
 static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
                 unsigned long end, int write, struct page **pages, int *nr)
 {
@@ -128,6 +138,8 @@ static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
         do {
                 VM_BUG_ON(compound_head(page) != head);
                 pages[*nr] = page;
+                if (PageTail(page))
+                        get_huge_page_tail(page);
                 (*nr)++;
                 page++;
                 refs++;