[036/143] mm/hugeltb: clarify (chg - freed) won't go negative in hugetlb_unreserve_pages()

Commit Message

Andrew Morton May 5, 2021, 1:34 a.m. UTC

From: Miaohe Lin <[e-mail]>
Subject: 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/[e-mail]
Signed-off-by: Miaohe Lin <[e-mail]>
Cc: Feilong Lin <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/hugetlb.c |    3 +++
 1 file changed, 3 insertions(+)

Patch

--- a/mm/hugetlb.c~mm-hugeltb-clarify-chg-freed-wont-go-negative-in-hugetlb_unreserve_pages
+++ a/mm/hugetlb.c
@@ -5267,6 +5267,9 @@  long hugetlb_unreserve_pages(struct inod
 	/*
 	 * 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);