Re: [PATCH] [v2] mm/vmscan: avoid false-positive -Wuninitialized warning

[Axel Rasmussen <axelrasmussen@google.com>]

On Mon, Apr 13, 2026 at 11:52 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> When the -fsanitize=bounds sanitizer is enabled, gcc-16 sometimes runs
> into a corner case in the read_ctrl_pos() pos function, where it sees
> possible undefined behavior from the 'tier' index overflowing, presumably
> in the case that this was called with a negative tier:
>
> In function 'get_tier_idx',
>     inlined from 'isolate_folios' at mm/vmscan.c:4671:14:
> mm/vmscan.c: In function 'isolate_folios':
> mm/vmscan.c:4645:29: error: 'pv.refaulted' is used uninitialized [-Werror=uninitialized]
>
> Part of the problem seems to be that read_ctrl_pos() has unusual calling
> conventions since commit 37a260870f2c ("mm/mglru: rework type selection")
> where passing MAX_NR_TIERS makes it accumulate all tiers but passing a
> smaller positive number makes it read a single tier instead.
>
> Shut up the warning by adding a fake initialization to the two instances
> of this variable that can run into that corner case.
>
> Link: https://lore.kernel.org/all/CAJHvVcjtFW86o5FoQC8MMEXCHAC0FviggaQsd5EmiCHP+1fBpg@mail.gmail.com/
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>

> ---
> v2: replace the earlier more invasive cleanup with a trivial
>     workaround
> ---
>  mm/vmscan.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d3312c51f3f2..f829435d2807 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4760,7 +4760,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  static int get_tier_idx(struct lruvec *lruvec, int type)
>  {
>         int tier;
> -       struct ctrl_pos sp, pv;
> +       struct ctrl_pos sp, pv = {};
>
>         /*
>          * To leave a margin for fluctuations, use a larger gain factor (2:3).
> @@ -4779,7 +4779,7 @@ static int get_tier_idx(struct lruvec *lruvec, int type)
>
>  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;
> --
> 2.39.5
>