On 2/27/20 2:56 AM, Andrew Morton wrote:
> On Wed, 26 Feb 2020 18:04:56 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
>> 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()
>>
>
> Why don't we instead remove the first check for NULL? Remove some dead
> code instead of adding some?
>
Sure, works too. Anyone creating a new caller and trying to pass NULL will
crash quickly.
----8<----
>From 3118d9bea8a0fcef294915220f824674fb039ec3 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 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.
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>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git mm/compaction.c mm/compaction.c
index 672d3c78c6ab..35c991fbf983 100644
--- mm/compaction.c
+++ mm/compaction.c
@@ -2310,8 +2310,7 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
.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.
*/
--
2.25.1
