mm/hugetlb: fix missing hugetlb_lock for resv uncharge

There is a recent report on UFFDIO_COPY over hugetlb:

https://lore.kernel.org/all/[email protected]/

350:	lockdep_assert_held(&hugetlb_lock);

Should be an issue in hugetlb but triggered in an userfault context, where
it goes into the unlikely path where two threads modifying the resv map
together.  Mike has a fix in that path for resv uncharge but it looks like
the locking criteria was overlooked: hugetlb_cgroup_uncharge_folio_rsvd()
will update the cgroup pointer, so it requires to be called with the lock
held.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 79aa925 ("hugetlb_cgroup: fix reservation accounting")
Signed-off-by: Peter Xu <[email protected]>
Reported-by: [email protected]
Reviewed-by: Mina Almasry <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: xzpeter

mm/hugetlb.c

@@ -3268,9 +3268,12 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 
 		rsv_adjust = hugepage_subpool_put_pages(spool, 1);
 		hugetlb_acct_memory(h, -rsv_adjust);
-		if (deferred_reserve)
+		if (deferred_reserve) {
+			spin_lock_irq(&hugetlb_lock);
 			hugetlb_cgroup_uncharge_folio_rsvd(hstate_index(h),
 					pages_per_huge_page(h), folio);
+			spin_unlock_irq(&hugetlb_lock);
+		}
 	}
 
 	if (!memcg_charge_ret)