The patch titled
Subject: mm: fix div by zero in bdi_ratio_from_pages
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-fix-div-by-zero-in-bdi_ratio_from_pages.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-fix-div-by-zero-in-bdi_ratio_from_pages.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Stefan Roesch <shr@xxxxxxxxxxxx>
Subject: mm: fix div by zero in bdi_ratio_from_pages
Date: Fri, 3 Jan 2025 17:20:37 -0800
During testing it has been detected, that it is possible to get div by
zero error in bdi_set_min_bytes. The error is caused by the function
bdi_ratio_from_pages(). bdi_ratio_from_pages() calls global_dirty_limits.
If the dirty threshold is 0, the div by zero is raised. This can happen
if the root user is setting:
echo 0 > /proc/sys/vm/dirty_ration.
The following is a test case:
echo 0 > /proc/sys/vm/dirty_ratio
cd /sys/class/bdi/<device>
echo 1 > strict_limit
echo 8192 > min_bytes
==> error is raised.
The problem is addressed by returning -EINVAL if dirty_ratio or
dirty_bytes is set to 0.
Link: https://lkml.kernel.org/r/20250104012037.159386-1-shr@xxxxxxxxxxxx
Reported-by: cheung wall <zzqq0103.hey@xxxxxxxxx>
Closes: https://lore.kernel.org/linux-mm/87pll35yd0.fsf@xxxxxxxxxxxx/T/#t
Signed-off-by: Stefan Roesch <shr@xxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Qiang Zhang <zzqq0103.hey@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page-writeback.c | 2 ++
1 file changed, 2 insertions(+)
--- a/mm/page-writeback.c~mm-fix-div-by-zero-in-bdi_ratio_from_pages
+++ a/mm/page-writeback.c
@@ -692,6 +692,8 @@ static unsigned long bdi_ratio_from_page
unsigned long ratio;
global_dirty_limits(&background_thresh, &dirty_thresh);
+ if (!dirty_thresh)
+ return -EINVAL;
ratio = div64_u64(pages * 100ULL * BDI_RATIO_SCALE, dirty_thresh);
return ratio;
_
Patches currently in -mm which might be from shr@xxxxxxxxxxxx are
mm-fix-div-by-zero-in-bdi_ratio_from_pages.patch
