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@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>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
--
Mel Gorman
SUSE Labs
