On 2/24/20 7:35 AM, Dan Carpenter wrote:
> Hello Mel Gorman,
>
> This is a semi-automatic email about new static checker warnings.
>
> 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)
>
----8<----
>From fb264bb52576a0582e8a92952cf91e8b61d105c3 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@xxxxxxx>
Date: Wed, 26 Feb 2020 17:53:54 +0100
Subject: [PATCH] mm, compaction: fully assume capture can be 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);
Hence no stable tag needed, but let's make this more robust and remove the
warning by assuming capture might be NULL in the second dereference.
Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git mm/compaction.c mm/compaction.c
index 672d3c78c6ab..f4bb8091cf4f 100644
--- mm/compaction.c
+++ mm/compaction.c
@@ -2318,8 +2318,10 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
VM_BUG_ON(!list_empty(&cc.freepages));
VM_BUG_ON(!list_empty(&cc.migratepages));
- *capture = capc.page;
- current->capture_control = NULL;
+ if (capture) {
+ *capture = capc.page;
+ current->capture_control = NULL;
+ }
return ret;
}
--
2.25.1
