FAILED: patch "[PATCH] mm/page-writeback: avoid 32-bit overflow in dirty threshold shares" failed to apply to 6.12-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 9597c2056e05bd79df9d343a9b42e84a54de1c52
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109200742-failed-6.12-9597c2056e05@kernel.org' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 9597c2056e05bd79df9d343a9b42e84a54de1c52 Mon Sep 17 00:00:00 2001
From: Hungyu Lin <dennylin0707@gmail.com>
Date: Wed, 21 Oct 2026 06:06:54 +0000
Subject: [PATCH] mm/page-writeback: avoid 32-bit overflow in dirty threshold
 shares

The ratios used by __wb_calc_thresh() are scaled to parts per million, but
the threshold and ratio operands are unsigned long.  On 32-bit kernels,
their multiplication can wrap before the result is assigned to u64.

For example, with a threshold of 25600 pages and the default maximum ratio
of 1000000, wb_max_thresh becomes 4125 pages instead of 25600.  The shared
threshold and minimum share calculations can overflow in the same way,
producing incorrect per-writeback dirty limits.

Use a u64 threshold for the three ratio multiplications and div_u64() to
retain the existing division order and rounding.

Link: https://lore.kernel.org/20261021060654.27855-1-dennylin0707@gmail.com
Fixes: ae82291e9ca4 ("mm: use part per 1000000 for bdi ratios")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Stefan Roesch <shr@devkernel.io>
Cc: <stable@vger.kernel.org>
---
 mm/page-writeback.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 499a35473e4f3..0694ec4098f6d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -883,6 +883,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
 {
 	struct wb_domain *dom = dtc_dom(dtc);
 	struct bdi_writeback *wb = dtc->wb;
+	u64 threshold = thresh;
 	u64 wb_thresh;
 	u64 wb_max_thresh;
 	unsigned long numerator, denominator;
@@ -894,13 +895,14 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
 	fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
 			      &numerator, &denominator);
 
-	wb_thresh = (thresh * (100 * BDI_RATIO_SCALE - bdi_min_ratio)) / (100 * BDI_RATIO_SCALE);
+	wb_thresh = div_u64(threshold * (100 * BDI_RATIO_SCALE - bdi_min_ratio),
+			    100 * BDI_RATIO_SCALE);
 	wb_thresh *= numerator;
 	wb_thresh = div64_ul(wb_thresh, denominator);
 
 	wb_min_max_ratio(wb, &wb_min_ratio, &wb_max_ratio);
 
-	wb_thresh += (thresh * wb_min_ratio) / (100 * BDI_RATIO_SCALE);
+	wb_thresh += div_u64(threshold * wb_min_ratio, 100 * BDI_RATIO_SCALE);
 
 	/*
 	 * It's very possible that wb_thresh is close to 0 not because the
@@ -916,7 +918,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
 			wb_thresh = max(wb_thresh, (thresh - dtc->dirty) / 8);
 	}
 
-	wb_max_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE);
+	wb_max_thresh = div_u64(threshold * wb_max_ratio, 100 * BDI_RATIO_SCALE);
 	if (wb_thresh > wb_max_thresh)
 		wb_thresh = wb_max_thresh;
 
-- 
2.53.0