From: Vlastimil Babka vbabka@suse.cz
commit 6467552ca64c4ddd2b83ed73192107d7145f533b upstream.
Dan reports:
The patch 5e1f0f098b46: "mm, compaction: capture a page under direct compaction" from Mar 5, 2019, leads to the following Smatch complaint:
mm/compaction.c:2321 compact_zone_order() error: we previously assumed 'capture' could be null (see line 2313)
mm/compaction.c 2288 static enum compact_result compact_zone_order(struct zone *zone, int order, 2289 gfp_t gfp_mask, enum compact_priority prio, 2290 unsigned int alloc_flags, int classzone_idx, 2291 struct page **capture) ^^^^^^^
2313 if (capture) ^^^^^^^ Check for NULL
2314 current->capture_control = &capc; 2315 2316 ret = compact_zone(&cc, &capc); 2317 2318 VM_BUG_ON(!list_empty(&cc.freepages)); 2319 VM_BUG_ON(!list_empty(&cc.migratepages)); 2320 2321 *capture = capc.page; ^^^^^^^^ Unchecked dereference.
2322 current->capture_control = NULL; 2323
In practice this is not an issue, as the only caller path passes non-NULL capture:
__alloc_pages_direct_compact() struct page *page = NULL; try_to_compact_pages(capture = &page); compact_zone_order(capture = capture);
So let's remove the unnecessary check, which should also make Smatch happy.
Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction") Reported-by: Dan Carpenter dan.carpenter@oracle.com Suggested-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Vlastimil Babka vbabka@suse.cz Signed-off-by: Andrew Morton akpm@linux-foundation.org Reviewed-by: Andrew Morton akpm@linux-foundation.org Acked-by: Mel Gorman mgorman@techsingularity.net Link: http://lkml.kernel.org/r/18b0df3c-0589-d96c-23fa-040798fee187@suse.cz Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/compaction.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/compaction.c +++ b/mm/compaction.c @@ -2310,8 +2310,7 @@ static enum compact_result compact_zone_ .page = NULL, };
- if (capture) - current->capture_control = &capc; + current->capture_control = &capc;
ret = compact_zone(&cc, &capc);
@@ -2333,6 +2332,7 @@ int sysctl_extfrag_threshold = 500; * @alloc_flags: The allocation flags of the current allocation * @ac: The context of current allocation * @prio: Determines how hard direct compaction should try to succeed + * @capture: Pointer to free page created by compaction will be stored here * * This is the main entry point for direct page compaction. */