mremap: check for overflow using deltas

Using "- 1" relies on the old_end to be page aligned and PAGE_SIZE > 1,
those are reasonable requirements but the check remains obscure and it
looks more like an off by one error than an overflow check.  This I feel
will improve readability.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Johannes Weiner <jweiner@redhat.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/mm/mremap.c b/mm/mremap.c
index 506fa44403df..195e866568e0 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -141,9 +141,10 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
         for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
                 cond_resched();
                 next = (old_addr + PMD_SIZE) & PMD_MASK;
-                if (next - 1 > old_end)
-                        next = old_end;
+                /* even if next overflowed, extent below will be ok */
                 extent = next - old_addr;
+                if (extent > old_end - old_addr)
+                        extent = old_end - old_addr;
                 old_pmd = get_old_pmd(vma->vm_mm, old_addr);
                 if (!old_pmd)
                         continue;