mm: hugetlb: initialize PG_reserved for tail pages of gigantic compound pages

Commit 11feeb4980 ("kvm: optimize away THP checks in
kvm_is_mmio_pfn()") introduced a memory leak when KVM is run on gigantic
compound pages.

That commit depends on the assumption that PG_reserved is identical for
all head and tail pages of a compound page.  So that if get_user_pages
returns a tail page, we don't need to check the head page in order to
know if we deal with a reserved page that requires different
refcounting.

The assumption that PG_reserved is the same for head and tail pages is
certainly correct for THP and regular hugepages, but gigantic hugepages
allocated through bootmem don't clear the PG_reserved on the tail pages
(the clearing of PG_reserved is done later only if the gigantic hugepage
is freed).

This patch corrects the gigantic compound page initialization so that we
can retain the optimization in 11feeb4980.  The cacheline was already
modified in order to set PG_tail so this won't affect the boot time of
large memory systems.

[akpm@linux-foundation.org: tweak comment layout and grammar]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: andy123 <ajs124.ajs124@gmail.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -696,8 +696,22 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
 	/* we rely on prep_new_huge_page to set the destructor */
 	set_compound_order(page, 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
+		 * hugepages and clear the PG_reserved bit from all tail pages
+		 * too.  Otherwse drivers using get_user_pages() to access tail
+		 * pages may get the reference counting wrong if they see
+		 * PG_reserved set on a tail page (despite the head page not
+		 * having PG_reserved set).  Enforcing this consistency between
+		 * head and tail pages allows drivers to optimize away a check
+		 * on the head page when they need know if put_page() is needed
+		 * after get_user_pages().
+		 */
+		__ClearPageReserved(p);
 		set_page_count(p, 0);
 		p->first_page = page;
 	}
@@ -1330,9 +1344,9 @@ static void __init gather_bootmem_prealloc(void)
 #else
 		page = virt_to_page(m);
 #endif
-		__ClearPageReserved(page);
 		WARN_ON(page_count(page) != 1);
 		prep_compound_huge_page(page, h->order);
+		WARN_ON(PageReserved(page));
 		prep_new_huge_page(h, page, page_to_nid(page));
 		/*
 		 * If we had gigantic hugepages allocated at boot time, we need