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

[Andrew Morton]

On Wed, 16 Sep 2026 10:34:46 +0200 Arnd Bergmann <arnd@xxxxxxxxxx> wrote:

> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> I previously worked around a false-postive gcc-16 warning in the
> get_tier_idx() function, by adding a fake initializer. This happens with
> the -fsanitize=bounds sanitizer when the compiler creates a specialized
> variant of isolate_folios():
> 
> In function 'get_tier_idx',
>     inlined from 'isolate_folios.constprop' at mm/vmscan.c:4982:9:
> mm/vmscan.c:4934:9: error: 'sp.refaulted' is used uninitialized [-Werror=uninitialized]
>  4934 |         read_ctrl_pos(lruvec, type, LRU_TIER_MIN, LRU_TIER_MIN, 2, &sp);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/vmscan.c: In function 'isolate_folios.constprop':
> mm/vmscan.c:4946:25: note: 'sp.refaulted' was declared here
>  4946 |         struct ctrl_pos sp, pv = {};
>       |                         ^~
> 
> Adding another "= {}" would solve the problem as well, but to prevent
> this from happening again after the next code refactoring, try instead to
> prevent this by forbidding interprocedural optimizations on this function.
> 
> ...
>
> --- 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.


--- a/mm/vmscan.c~mm-vmscan-avoid-false-positive-wuninitialized-warning-again
+++ a/mm/vmscan.c
@@ -3195,8 +3195,12 @@ struct ctrl_pos {
 	int gain;
 };
 
-static void read_ctrl_pos(struct lruvec *lruvec, int type, int tier, int gain,
-			  struct ctrl_pos *pos)
+/*
+ * __noipa works around  gcc-16 warning for uninitialized use of
+ * pos->refaulted
+ */
+static __noipa void read_ctrl_pos(struct lruvec *lruvec, int type, int tier,
+			  int gain, struct ctrl_pos *pos)
 {
 	int i;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
@@ -4802,7 +4806,7 @@ static int scan_folios(unsigned long nr_
 static int get_tier_idx(struct lruvec *lruvec, int type)
 {
 	int tier;
-	struct ctrl_pos sp, pv = {};
+	struct ctrl_pos sp, pv;
 
 	/*
 	 * To leave a margin for fluctuations, use a larger gain factor (2:3).
@@ -4821,7 +4825,7 @@ static int get_tier_idx(struct lruvec *l
 
 static int get_type_to_scan(struct lruvec *lruvec, int swappiness)
 {
-	struct ctrl_pos sp, pv = {};
+	struct ctrl_pos sp, pv;
 
 	if (swappiness <= MIN_SWAPPINESS + 1)
 		return LRU_GEN_FILE;
_