mm/cma: fix alloc_contig_range ret code/potential leak

mm/cma: fix alloc_contig_range ret code/potential leak

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 the 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.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 8ef5849fa8a2 ("mm/cma: always check which page caused allocation failure")
Signed-off-by: Mike Kravetz <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Joonsoo Kim <[email protected]>
Cc: Michal Nazarewicz <[email protected]>
Cc: Laura Abbott <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>