mm/hugeltb: clarify (chg - freed) won't go negative in hugetlb_unreserve_pages()

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.

Link: https://lkml.kernel.org/r/20210410072348.20437-4-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Feilong Lin <linfeilong@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d91bd780ba98f1a6e8a7ad1b3cf3cc0d7ed4d4d7..b1525ae1bf1f7a668276737eb0f6e1a64dec33a0 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5267,6 +5267,9 @@ 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);