[merged mm-hotfixes-stable] mm-page_alloc-make-percpu_pagelist_high_fraction-reads-lock-free.patch removed from -mm tree

[merged mm-hotfixes-stable] mm-page_alloc-make-percpu_pagelist_high_fraction-reads-lock-free.patch removed from -mm tree

[Andrew Morton]

The quilt patch titled
     Subject: mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free
has been removed from the -mm tree.  Its filename was
     mm-page_alloc-make-percpu_pagelist_high_fraction-reads-lock-free.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Aboorva Devarajan <aboorvad@linux.ibm.com>
Subject: mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free
Date: Mon, 1 Dec 2025 11:30:09 +0530

When page isolation loops indefinitely during memory offline, reading
/proc/sys/vm/percpu_pagelist_high_fraction blocks on pcp_batch_high_lock,
causing hung task warnings.

Make procfs reads lock-free since percpu_pagelist_high_fraction is a
simple integer with naturally atomic reads, writers still serialize via
the mutex.

This prevents hung task warnings when reading the procfs file during
long-running memory offline operations.

[akpm@linux-foundation.org: add comment, per Michal]
  Link: https://lkml.kernel.org/r/aS_y9AuJQFydLEXo@tiehlicka
Link: https://lkml.kernel.org/r/20251201060009.1420792-1-aboorvad@linux.ibm.com
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/mm/page_alloc.c~mm-page_alloc-make-percpu_pagelist_high_fraction-reads-lock-free
+++ a/mm/page_alloc.c
@@ -6667,11 +6667,19 @@ static int percpu_pagelist_high_fraction
 	int old_percpu_pagelist_high_fraction;
 	int ret;
 
+	/*
+	 * Avoid using pcp_batch_high_lock for reads as the value is read
+	 * atomically and a race with offlining is harmless.
+	 */
+
+	if (!write)
+		return proc_dointvec_minmax(table, write, buffer, length, ppos);
+
 	mutex_lock(&pcp_batch_high_lock);
 	old_percpu_pagelist_high_fraction = percpu_pagelist_high_fraction;
 
 	ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
-	if (!write || ret < 0)
+	if (ret < 0)
 		goto out;
 
 	/* Sanity checking to avoid pcp imbalance */
_

Patches currently in -mm which might be from aboorvad@linux.ibm.com are