[PATCH v2 3/5] mm/hugeltb: clarify (chg - freed) won't go negative in hugetlb_unreserve_pages()

[Miaohe Lin <linmiaohe@huawei.com>]

The resv_map could be NULL since this routine can be called in the evict
inode path for all hugetlbfs inodes and we will have chg = 0 in this case.
But (chg - freed) won't go negative as Mike pointed out:

 "If resv_map is NULL, then no hugetlb pages can be allocated/associated
  with the file.  As a result, remove_inode_hugepages will never find any
  huge pages associated with the inode and the passed value 'freed' will
  always be zero."

Add a comment clarifying this to make it clear and also avoid confusion.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/hugetlb.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9b4c05699a90..387c9a62615e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5435,6 +5435,9 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
 	/*
 	 * If the subpool has a minimum size, the number of global
 	 * reservations to be released may be adjusted.
+	 *
+	 * Note that !resv_map implies freed == 0. So (chg - freed)
+	 * won't go negative.
 	 */
 	gbl_reserve = hugepage_subpool_put_pages(spool, (chg - freed));
 	hugetlb_acct_memory(h, -gbl_reserve);
-- 
2.19.1