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

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

[Arnd Bergmann]

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>
---
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