+ mm-damon-core-handle-extreme-memory-state-in-get_in_active_mem_bp.patch added to mm-new branch

+ mm-damon-core-handle-extreme-memory-state-in-get_in_active_mem_bp.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/damon/core: handle extreme memory state in get_in_active_mem_bp()
has been added to the -mm mm-new branch.  Its filename is
     mm-damon-core-handle-extreme-memory-state-in-get_in_active_mem_bp.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-core-handle-extreme-memory-state-in-get_in_active_mem_bp.patch

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

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: SJ Park <sj@kernel.org>
Subject: mm/damon/core: handle extreme memory state in get_in_active_mem_bp()
Date: Tue, 1 Sep 2026 06:18:49 -0700

damos_get_in_active_mem_bp() uses the sum of the active and inactive
memory amount as a denominator.  In an extreme and unlikely environment,
active and inactive memory might be zero.  In this case, hence, it results
in a divide by zero problem.  Avoid it by changing the denominator to one
if it is zero, before it is being used.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260901131850.98037-9-sj@kernel.org
Link: https://lore.kernel.org/20260721034756.147011-1-sj@kernel.org [1]
Fixes: 4835e2871321 ("mm/damon/core: introduce [in]active memory ratio damos quota goal metric")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: <stable@vger.kernel.org> # 7.0.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/damon/core.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/damon/core.c~mm-damon-core-handle-extreme-memory-state-in-get_in_active_mem_bp
+++ a/mm/damon/core.c
@@ -3006,7 +3006,7 @@ static unsigned int damos_get_in_active_
 		global_node_page_state(NR_LRU_BASE + LRU_ACTIVE_FILE);
 	inactive = global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_ANON) +
 		global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_FILE);
-	total = active + inactive;
+	total = max(active + inactive, 1);
 	if (active_ratio)
 		return mult_frac(active, 10000, total);
 	return mult_frac(inactive, 10000, total);
_

Patches currently in -mm which might be from sj@kernel.org are

mm-damon-core-skip-applying-scheme-if-region-split-for-quota-fails.patch
mm-damon-paddr-respect-folio-end-for-damos_stat.patch
mm-damon-paddr-respect-folio-end-for-damos-actions-except-stat.patch
mm-damon-vaddr-respect-folio-end-for-damos_stat.patch
mm-damon-vaddr-respect-folio-end-for-damos_migrate_hotcold.patch
mm-damon-core-handle-extreme-memory-state-in-damon_get_node_mem_bp.patch
mm-damon-core-handle-extreme-memory-state-in-get_node_memcg_used_bp.patch
mm-damon-core-handle-extreme-memory-state-in-get_in_active_mem_bp.patch