+ mm-stop-recalculating-min_free_kbytes-once-set-by-userspace.patch added to mm-unstable branch

+ mm-stop-recalculating-min_free_kbytes-once-set-by-userspace.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: stop recalculating min_free_kbytes once set by userspace
has been added to the -mm mm-unstable branch.  Its filename is
     mm-stop-recalculating-min_free_kbytes-once-set-by-userspace.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-stop-recalculating-min_free_kbytes-once-set-by-userspace.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: Nimrod Oren <noren@nvidia.com>
Subject: mm: stop recalculating min_free_kbytes once set by userspace
Date: Wed, 14 Oct 2026 17:08:59 +0300

min_free_kbytes is recalculated on memory hotplug and on THP sysfs writes.
The new value comes from calculate_min_free_kbytes() or, while THP is
enabled, from set_recommended_min_free_kbytes().  A value written to the
vm.min_free_kbytes sysctl is replaced whenever the new value is higher, so
a lower value does not stick.  This matters most when THP is enabled on
configurations with large pageblocks, where the new value can be excessive
[1].

For example, with default arm64 pageblock sizes, the THP recommendation
can reach 5.5 GiB per eligible zone with 64 KiB pages vs 22 MiB with 4 KiB
pages.  This keeps a significant amount of memory free, reducing what
applications can use and triggering reclaim much earlier.  Users can lower
vm.min_free_kbytes to avoid this, but the lower value only sticks until
the next recalculation.

Stop recalculating min_free_kbytes once a value has been written to the
sysctl.  Systems where the sysctl is never written keep the current
behavior.

Link: https://lore.kernel.org/20261014140859.1006967-1-noren@nvidia.com
Link: https://lore.kernel.org/r/20260716173504.760369-1-noren@nvidia.com/ [1]
Signed-off-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Link: https://lore.kernel.org/r/d202dcc3-d7d7-4e98-a9a5-3137cfbfd041@kernel.org/
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: David Laight <david.laight.linux@gmail.com>
---

 Documentation/admin-guide/sysctl/vm.rst |    7 +++++++
 mm/khugepaged.c                         |    7 +------
 mm/page_alloc.c                         |    6 +-----
 3 files changed, 9 insertions(+), 11 deletions(-)

--- a/Documentation/admin-guide/sysctl/vm.rst~mm-stop-recalculating-min_free_kbytes-once-set-by-userspace
+++ a/Documentation/admin-guide/sysctl/vm.rst
@@ -539,6 +539,13 @@ watermark[WMARK_MIN] value for each lowm
 Each lowmem zone gets a number of reserved free pages based
 proportionally on its size.
 
+By default, the kernel sets this value based on the amount of lowmem
+and recalculates it on memory hotplug.  When Transparent Hugepage (THP)
+support is enabled through global or per-size controls, the kernel may
+also raise it to help keep pageblocks free and reduce fragmentation for
+THP allocations.  Once a value is written to this file, the kernel no
+longer adjusts it automatically.
+
 Some minimal amount of memory is needed to satisfy PF_MEMALLOC
 allocations; if you set this to lower than 1024KB, your system will
 become subtly broken, and prone to deadlock under high loads.
--- a/mm/khugepaged.c~mm-stop-recalculating-min_free_kbytes-once-set-by-userspace
+++ a/mm/khugepaged.c
@@ -3148,13 +3148,8 @@ void set_recommended_min_free_kbytes(voi
 			      (unsigned long) nr_free_buffer_pages() / 20);
 	recommended_min <<= (PAGE_SHIFT-10);
 
-	if (recommended_min > min_free_kbytes) {
-		if (user_min_free_kbytes >= 0)
-			pr_info_ratelimited("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations\n",
-					    min_free_kbytes, recommended_min);
-
+	if (user_min_free_kbytes < 0 && recommended_min > min_free_kbytes)
 		min_free_kbytes = recommended_min;
-	}
 
 update_wmarks:
 	setup_per_zone_wmarks();
--- a/mm/page_alloc.c~mm-stop-recalculating-min_free_kbytes-once-set-by-userspace
+++ a/mm/page_alloc.c
@@ -6737,12 +6737,8 @@ void calculate_min_free_kbytes(void)
 	lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
 	new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
 
-	if (new_min_free_kbytes > user_min_free_kbytes)
+	if (user_min_free_kbytes < 0)
 		min_free_kbytes = clamp(new_min_free_kbytes, 128, 262144);
-	else
-		pr_warn_ratelimited("min_free_kbytes is not updated to %d because user defined value %d is preferred\n",
-				    new_min_free_kbytes, user_min_free_kbytes);
-
 }
 
 int __meminit init_per_zone_wmark_min(void)
_

Patches currently in -mm which might be from noren@nvidia.com are

mm-stop-recalculating-min_free_kbytes-once-set-by-userspace.patch