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mm: stop recalculating min_free_kbytes once set by userspace

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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>
This commit is contained in:
Nimrod Oren 2026-10-14 17:08:59 +03:00 committed by Andrew Morton
parent 7135a05838
commit 397e087605
3 changed files with 9 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
Documentation/admin-guide/sysctl/vm.rst
View File

@ -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.

7
mm/khugepaged.c
View File

@ -3144,13 +3144,8 @@ 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();

6
mm/page_alloc.c
View File

@ -6722,12 +6722,8 @@ 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)