mm,hugetlb: take hugetlb_lock before decrementing h->resv_huge_pages

The h->*_huge_pages counters are protected by the hugetlb_lock, but
alloc_huge_page has a corner case where it can decrement the counter
outside of the lock.

This could lead to a corrupted value of h->resv_huge_pages, which we have
observed on our systems.

Take the hugetlb_lock before decrementing h->resv_huge_pages to avoid a
potential race.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: a88c769 ("mm: hugetlb: fix hugepage memory leak caused by wrong reserve count")
Signed-off-by: Rik van Riel <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Glen McCready <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/hugetlb.c

@@ -2924,11 +2924,11 @@ struct page *alloc_huge_page(struct vm_area_struct *vma,
 		page = alloc_buddy_huge_page_with_mpol(h, vma, addr);
 		if (!page)
 			goto out_uncharge_cgroup;
+		spin_lock_irq(&hugetlb_lock);
 		if (!avoid_reserve && vma_has_reserves(vma, gbl_chg)) {
 			SetHPageRestoreReserve(page);
 			h->resv_huge_pages--;
 		}
-		spin_lock_irq(&hugetlb_lock);
 		list_add(&page->lru, &h->hugepage_activelist);
 		set_page_refcounted(page);
 		/* Fall through */