Re: [bug report] mm, compaction: capture a page under direct compaction

[Vlastimil Babka <vbabka@suse.cz>]

On 2/27/20 2:56 AM, Andrew Morton wrote:
> On Wed, 26 Feb 2020 18:04:56 +0100 Vlastimil Babka <vbabka@suse.cz> 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@suse.cz>
>> 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@suse.cz>
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@oracle.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
---
 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