Re: [PATCH] mm/vmscan: avoid false-positive -Wuninitialized warning, again

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 17 Sep 2026 07:50:43 +0200 "Arnd Bergmann" <arnd@arndb.de> wrote:

> On Thu, Sep 17, 2026, at 01:30, Andrew Morton wrote:
> > On Wed, 16 Sep 2026 16:28:16 -0700 Andrew Morton  <akpm@linux-foundation.org> wrote:
> >> >
> >> > --- a/mm/vmscan.c
> >> > +++ b/mm/vmscan.c
> >> > @@ -3274,8 +3274,10 @@ struct ctrl_pos {
> >> >  	int gain;
> >> >  };
> >> >  
> >> > -static void read_ctrl_pos(struct lruvec *lruvec, int type, int tier_min,
> >> > -			  int tier_max, int gain, struct ctrl_pos *pos)
> >> > +/* __noipa works around  gcc-16 warning for uninitizled use of pos->refaulted */
> >> > +static __noipa void read_ctrl_pos(struct lruvec *lruvec, int type,
> >> > +				  int tier_min, int tier_max, int gain,
> >> > +				  struct ctrl_pos *pos)
> >> >  {
> >> >  	int i;
> >> >  	struct lru_gen_folio *lrugen = &lruvec->lrugen;
> >> 
> >> Current code has changed here somewhat, but I expect the error is still
> >> there.  I fixed that "uninitizled" while in there.  Altered patch is
> >> below.
> >
> >
> > Then the build blew up in unexpected ways.  gcc-15.2.0.
> >
> > The failure looks like a legit min() signedness thing which has been
> > there quite a while.  I'm thinking that __noipa surfaced this for some
> > reason?
> 
> Right, I now saw the same thing here on the current linux-next.
> 
> What I found now is that the __noipa is only needed on top of 
> "mm/mglru: use explicit tier range in read_ctrl_pos()", which was
> in next-20260915 but disappeared in next-20260916. This patch
> also removed the min().

So we don't need cc:stable?

> I think the reason why __noipa causes the warning about min() is
> that it prevents the constant propagation into read_ctrl_pos() and
> in turn the hack that suppresses warning about mixed types in
> minmax.h when both sides are constant.
> 
> Is the mglru series currently expected to make it into 7.4?

Yes, "mm/mglru: use explicit tier range in read_ctrl_pos()" is in
mm-unstable at present.

> If not, I would withdraw my __noipa and hope that the next
> round of changes to read_ctrl_pos() does not run into this
> problem again.

OK, I'll drop "mm/vmscan.c: fix min() signedness mismatch" and shall
rely on "mm/mglru: use explicit tier range in read_ctrl_pos()" to fix
the min() thing.

And I'll stage "mm/vmscan: avoid false-positive -Wuninitialized
warning, again".  ahead of "mm/mglru: use explicit tier range in
read_ctrl_pos()" to fix the build glitch wihout a bisection hole.

Does that sound sane?