From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: mm, compaction: fully assume capture is not NULL in compact_zone_order()
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.
Link: http://lkml.kernel.org/r/18b0df3c-0589-d96c-23fa-040798fee187@xxxxxxx
Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/compaction.c~mm-compaction-fully-assume-capture-is-not-null-in-compact_zone_order
+++ a/mm/compaction.c
@@ -2314,8 +2314,7 @@ static enum compact_result compact_zone_
.page = NULL,
};
- if (capture)
- current->capture_control = &capc;
+ current->capture_control = &capc;
ret = compact_zone(&cc, &capc);
@@ -2337,6 +2336,7 @@ 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.
*/
_
