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

[Nimrod Oren <noren@nvidia.com>]

On 15/10/2026 0:48, Andrew Morton wrote:
> On Wed, 14 Oct 2026 17:08:59 +0300 Nimrod Oren <noren@nvidia.com> wrote:
> 
>> 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.
> 
> This problem lies somewhere in the range of "minor annoyance" to
> "catastrophically horrible", but it isn't clear where in that range it
> falls.
> 
> So can we please can we have a paragraph which explains the real-world
> userspace-visible runtime effects of this problem?

Sure. Can we add the following after the first paragraph?

  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.

Thanks