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

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Feb 27, 2020 at 08:45:46AM +0100, Vlastimil Babka wrote:

Bit late to the game but;

> ----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>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs