Re: [PATCH] mm, compaction: avoid uninitialized variable use

Arnd Bergmann
2016-05-11T14:58:23Z

On Wednesday 11 May 2016 16:44:07 Michal Hocko wrote:
> On Wed 11-05-16 15:24:44, Arnd Bergmann wrote:
> > A recent rework of the compaction code introduced a warning about
> > an uninitialized variable when CONFIG_COMPACTION is disabled and
> > __alloc_pages_direct_compact() does not set its 'compact_result'
> > output argument:
> >
> > mm/page_alloc.c: In function '__alloc_pages_nodemask':
> > mm/page_alloc.c:3651:6: error: 'compact_result' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> >
> > This adds another check for CONFIG_COMPACTION to ensure we never
> > evaluate the uninitialized variable in this configuration, which
> > is probably the simplest way to avoid the warning.
>
> I think that hiding this into __alloc_pages_direct_compact is a better
> idea. See the diff below

Ok, sounds good.

> ---
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4950d01ff935..14e3b4d93adc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3300,6 +3300,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> unsigned int alloc_flags, const struct alloc_context *ac,
> enum migrate_mode mode, enum compact_result *compact_result)
> {
> + *compact_result = COMPACT_DEFERRED;
> return NULL;
> }
>

I thought about this but didn't know which COMPACT_* value was appropriate here.

The behavior then changes a bit with your approach compared to mine,
because

if (compact_result == COMPACT_DEFERRED)
goto nopage;

is true now. I assume this is what we want though.

Arnd