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()
>
> 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.
>
> ...
>
> --- 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;
> }
Why don't we instead remove the first check for NULL? Remove some dead
code instead of adding some?
