[PATCH] mm: stop recalculating min_free_kbytes once set by userspace

[Nimrod Oren]

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].

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/r/20260716173504.760369-1-noren@xxxxxxxxxx/ [1]
Suggested-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
Link: https://lore.kernel.org/r/d202dcc3-d7d7-4e98-a9a5-3137cfbfd041@xxxxxxxxxx/
Signed-off-by: Nimrod Oren <noren@xxxxxxxxxx>
---
 Documentation/admin-guide/sysctl/vm.rst | 7 +++++++
 mm/khugepaged.c                         | 7 +------
 mm/page_alloc.c                         | 6 +-----
 3 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 5b318d17aa4b..189fc57dac5f 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -539,6 +539,13 @@ watermark[WMARK_MIN] value for each lowmem zone in the system.
 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.
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index dbae78b68dd8..3fd209f9377d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -3148,13 +3148,8 @@ void set_recommended_min_free_kbytes(void)
 			      (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();
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6ed36ba4d055..57a33da2562d 100644
--- a/mm/page_alloc.c
+++ b/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)
-- 
2.45.0