mm, hugetlb: close race when setting PageTail for gigantic pages

Now that gigantic pages are dynamically allocatable, care must be taken to
ensure that p->first_page is valid before setting PageTail.

If this isn't done, then it is possible to race and have compound_head()
return NULL.

Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0a9ac6c26832..c41b2a0ee273 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -917,7 +917,6 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
         __SetPageHead(page);
         __ClearPageReserved(page);
         for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
-                __SetPageTail(p);
                 /*
                  * For gigantic hugepages allocated through bootmem at
                  * boot, it's safer to be consistent with the not-gigantic
@@ -933,6 +932,9 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
                 __ClearPageReserved(p);
                 set_page_count(p, 0);
                 p->first_page = page;
+                /* Make sure p->first_page is always valid for PageTail() */
+                smp_wmb();
+                __SetPageTail(p);
         }
 }