FAILED: patch "[PATCH] mm/damon/core: skip applying scheme if region split for quota fails" failed to apply to 6.18-stable tree

FAILED: patch "[PATCH] mm/damon/core: skip applying scheme if region split for quota fails" failed to apply to 6.18-stable tree

[Sasha Levin]

The patch below does not apply to the 6.18-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-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 90c2fa6adfed7abe0935506a43cdff3a05c1fff0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107050847-failed-6.18-90c2fa6adfed@kernel.org' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Thanks,
Sasha

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

From 90c2fa6adfed7abe0935506a43cdff3a05c1fff0 Mon Sep 17 00:00:00 2001
From: SJ Park <sj@kernel.org>
Date: Tue, 1 Sep 2026 06:18:42 -0700
Subject: [PATCH] mm/damon/core: skip applying scheme if region split for quota
 fails

Patch series "mm/damon: fix DAMOS bugs in core, paddr and vaddr".

Fix misc bugs of DAMOS.  Patch 1 makes DAMOS less stress memory allocator
under extreme situation.  Patches 2 and 3 fix wrong folios walking in
DAMON_PADDR.  Patches 4 and 5 fix wrong folios walking in DAMON_VADDR.
Patches 6-8 handle extreme and unlikely memory situations that can cause
divide by zero and underflow.

All bugs are discovered by Sashiko.


This patch (of 8):

damos_apply_scheme() splits a region and apply the action to the subregion
if it is needed for not violating the quota.  The split operation
(damon_split_region_at()) could fail for allocation failure.  In the case,
the quota could be violated.  From the user's perspective, DAMOS becomes
more aggressive than expected under the extreme situation.  Handle the
failure.

The user impact is not critical.  The failure of damon_split_region_at()
is unlikely since it is arguably too small to fail.  Also DAMOS being
aggressive is limited to the single region.  Users can set min_nr_regions
to set the maximum size of each region.  If it is reasonably set, the
transient overhead shouldn't be critical.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]
Fixes: 2b8a248d5873 ("mm/damon/schemes: implement size quota for schemes application speed control")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: <stable@vger.kernel.org> # 5.16.x
---
 mm/damon/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index d63d4c6fd3ffd..53952d16ebd88 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2607,7 +2607,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 					c->min_region_sz);
 			if (!sz)
 				goto update_stat;
-			damon_split_region_at(t, r, sz);
+			if (damon_split_region_at(t, r, sz))
+				goto update_stat;
 		}
 		if (damos_core_filter_out(c, t, r, s))
 			return;
-- 
2.53.0