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

[Baoquan He <baoquan.he@linux.dev>]

On 09/16/26 at 04:30pm, 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?
> 
>   CC      mm/vmscan.o
> In file included from <command-line>:
> mm/vmscan.c: In function 'read_ctrl_pos':
> ././include/linux/compiler_types.h:702:45: error: call to '__compiletime_assert_798' declared with attribute error: min(tier, 4U - 1) signedness error
>   702 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>       |                                             ^
> 
> I'll queue a fix to switch this to min_t (yuck) for now.  It would of
> course be better to use more appropriate types in this code.  Unless
> tiers can be negative!

I drafted one to change 'tier' as 'unsigned int'. While in mm-new this
issue has gone away because Kairui's patch change 'tier' to 'tier_min'
and 'tier_max' and remove min().

commit b447513eee2f ("mm/mglru: use explicit tier range in read_ctrl_pos()")

I personally think your one line change is better for backporting to
stable tree.

From: Baoquan He <hebaoquan@kylinos.cn>
Subject: [PATCH] mm/vmscan: fix min() signedness mismatch in read_ctrl_pos()

read_ctrl_pos() declares 'tier' as int, but MAX_NR_TIERS is 4U, so
min(tier, MAX_NR_TIERS - 1) compares int with unsigned int and min()
reports a signedness error:

  error: min(tier, 4U - 1) signedness error

'tier' is never negative, it is a tier index or MAX_NR_TIERS to mean
all tiers. Make it unsigned int, then min() compares two unsigned
values and no min_t() needed.

Signed-off-by: Baoquan He <hebaoquan@kylinos.cn>
---
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f11491ee9ed5..1844d92a6732 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3195,10 +3195,10 @@ struct ctrl_pos {
 	int gain;
 };
 
-static void read_ctrl_pos(struct lruvec *lruvec, int type, int tier, int gain,
-			  struct ctrl_pos *pos)
+static void read_ctrl_pos(struct lruvec *lruvec, int type, unsigned int tier,
+			  int gain, struct ctrl_pos *pos)
 {
-	int i;
+	unsigned int i;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 	int hist = lru_hist_from_seq(lrugen->min_seq[type]);
 
@@ -4801,7 +4801,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 
 static int get_tier_idx(struct lruvec *lruvec, int type)
 {
-	int tier;
+	unsigned int tier;
 	struct ctrl_pos sp, pv = {};
 
 	/*