[PATCH v2] mm/cma: fix alloc_contig_range ret code/potential leak

[Mike Kravetz]

If the call __alloc_contig_migrate_range() in alloc_contig_range
returns -EBUSY, processing continues so that test_pages_isolated()
is called where there is a tracepoint to identify the busy pages.
However, it is possible for busy pages to become available between
the calls to these two routines.  In this case, the range of pages
may be allocated.   Unfortunately, the original return code (ret
== -EBUSY) is still set and returned to the caller.  Therefore,
the caller believes the pages were not allocated and they are leaked.

Update comment to indicate that allocation is still possible even if
__alloc_contig_migrate_range returns -EBUSY.  Also, clear return code
in this case so that it is not accidentally used or returned to caller.

Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/page_alloc.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 77e4d3c5c57b..25e81844d1aa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7582,11 +7582,18 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 
 	/*
 	 * In case of -EBUSY, we'd like to know which page causes problem.
-	 * So, just fall through. We will check it in test_pages_isolated().
+	 * So, just fall through. test_pages_isolated() has a tracepoint
+	 * which will report the busy page.
+	 *
+	 * It is possible that busy pages could become available before
+	 * the call to test_pages_isolated, and the range will actually be
+	 * allocated.  So, if we fall through be sure to clear ret so that
+	 * -EBUSY is not accidentally used or returned to caller.
 	 */
 	ret = __alloc_contig_migrate_range(&cc, start, end);
 	if (ret && ret != -EBUSY)
 		goto done;
+	ret =0;
 
 	/*
 	 * Pages from [start, end) are within a MAX_ORDER_NR_PAGES
-- 
2.13.6