linux-kernel - Re: [PATCH] mm/ksm.c - Fix compile warnings (PS ignore my formatting in previous email)

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Date:	Mon, 09 Nov 2009 19:15:31 +0530
From:	askb <askb23@...il.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	wharms@....de, Izik Eidus <ieidus@...hat.com>,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] mm/ksm.c - Fix compile warnings (PS ignore my
 formatting in previous email)

On Mon, 2009-11-09 at 10:51 +0000, Hugh Dickins wrote:
> On Wed, 4 Nov 2009, askb wrote:
> > On Wed, 2009-11-04 at 10:57 +0100, walter harms wrote:
> > > 
> > > askb schrieb:
> > > > Found the compiler warning on linux-next: 	
> > > > 
> > > > mm/ksm.c: In function ‘ksm_scan_thread’:
> > > > mm/ksm.c:1083: warning: ‘page2[0u]’ may be used uninitialized in this
> > > > function
> > > > mm/ksm.c:1083: note: ‘page2[0u]’ was declared here
> > > > 
> > > > fix for the above warning:
> > > > 
> > > > Signed-off-by: Anil SB <askb23@...il.com>
> > > > 
> > > > diff --git a/mm/ksm.c b/mm/ksm.c
> > > > index bef1af4..2ea0fd3 100644
> > > > --- a/mm/ksm.c
> > > > +++ b/mm/ksm.c
> > > > @@ -1080,7 +1080,7 @@ static void stable_tree_append(struct rmap_item *rmap_item,
> > > >   */
> > > >  static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
> > > >  {
> > > > -	struct page *page2[1];
> > > > +	struct page *page2[1] = {NULL};
> > > >  	struct rmap_item *tree_rmap_item;
> > > >  	unsigned int checksum;
> > > >  	int err;
> > > > 
> > > > 
> > > 
> > > a 1 element array looks strange, did you look what kind of magic is here used ?
> > > 
> > >From my understanding, it is used for place holder to check and return
> > an identical page from stable_tree_search(). Can we do with a double
> > indirection to struct page instead of the above? 
> > PS point me in the right direction.
> 
> I don't particularly like things like page2[1] myself, but there's
> nothing actually wrong with it, so no urgency to change it.
> 
> mm/ksm.c is under active development (better Cc the people involved
> with a sourcefile when considering changes to it), and I have a
> collection of patches under testing, which do change some names
> around here (along with more serious changes).  So, thanks for looking
> at this, but I'd prefer to avoid the interference of a trivial patch
> at the moment - sorry.
> 
> Which version of the compiler gave you that uninitialized warning?

gcc version 4.3.2 [gcc-4_3-branch revision 141291] (SUSE Linux)

> A version which lots of people are using, or just some snapshot?
> I've never seen a warning there myself, but they do keep "refining"
> that logic.
> 
I got the warning while doing "make randconfig". However, now after a
cleanup and redoing the same, the warning is not seen.

> Hugh's hypothesis: for every variable x initialized by a subfunction,
> there exists at least one version V of gcc, such that V reports that
> x may be used uninitialized.
> 
> Hugh

Thanks.

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