mm: stop recalculating min_free_kbytes once set by userspace

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>

0 files changed, 0 insertions, 0 deletions