+ mm-page-writeback-avoid-32-bit-overflow-in-dirty-threshold-shares.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/page-writeback: avoid 32-bit overflow in dirty threshold shares
has been added to the -mm mm-unstable branch.  Its filename is
     mm-page-writeback-avoid-32-bit-overflow-in-dirty-threshold-shares.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page-writeback-avoid-32-bit-overflow-in-dirty-threshold-shares.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Hungyu Lin <dennylin0707@gmail.com>
Subject: mm/page-writeback: avoid 32-bit overflow in dirty threshold shares
Date: Wed, 21 Oct 2026 06:06:54 +0000

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(-)

--- a/mm/page-writeback.c~mm-page-writeback-avoid-32-bit-overflow-in-dirty-threshold-shares
+++ a/mm/page-writeback.c
@@ -883,6 +883,7 @@ static unsigned long __wb_calc_thresh(st
 {
 	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(st
 	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(st
 			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;
 
_

Patches currently in -mm which might be from dennylin0707@gmail.com are

mm-page-writeback-avoid-32-bit-overflow-in-dirty-threshold-shares.patch
compat-avoid-double-rounding-in-times.patch