Re: [PATCH v2] mm/khugepaged: cap min_free_kbytes recommendation at 1 GiB

[Zi Yan]

On 31 Aug 2026, at 5:34, Lorenzo Stoakes (ARM) wrote:

> On Mon, Aug 31, 2026 at 11:20:30AM +0200, Michal Hocko wrote:
>> On Mon 31-08-26 10:56:35, Nimrod Oren wrote:
>>> When THP is enabled, set_recommended_min_free_kbytes() may raise
>>> min_free_kbytes using a heuristic that scales with pageblock_nr_pages.
>>> With MIGRATE_PCPTYPES equal to 3, the formula accounts for 11
>>> pageblocks for every eligible populated zone before capping the result
>>> at 5% of low memory.
>>>
>>> This is reasonable when a pageblock is 2 MiB, as on common 4 KiB page
>>> configurations, but scales poorly with larger base page sizes. With
>>> the default arm64 pageblock sizes, the contribution per eligible zone
>>> before the 5% cap is:
>>>
>>>   4 KiB pages:   2 MiB pageblock,   22 MiB per zone
>>>  16 KiB pages:  32 MiB pageblock,  352 MiB per zone
>>>  64 KiB pages: 512 MiB pageblock,  5.5 GiB per zone
>>>
>>> Consequently, min_free_kbytes can reach excessive and unwanted levels.
>>>
>>> Add an absolute 1 GiB cap to the recommendation, in addition to the
>>> existing percentage cap. This bounds the automatic recommendation to a
>>> sane value on systems with large pageblocks while preserving existing
>>> behavior for typical systems with 2 MiB pageblocks.
>>
>> I would argue that the whole model of increasing min_free_kbytes for THP
>> is wrong. This will trigger memory reclaim sooner and make the THP
>> availability more likely but this was at times where we didn't have
>> pro-active compaction and many changes in the compaction. So is this
>> actually needed in general resp. only large pageblocks systems?
>
> Ohh even better :)
>
> I did find it strange that we increased accordingly.
>
> I'm more than happy to see this just die altogether if people agree that's
> a sensible way forward...

It sounds reasonable to me.

My first reaction was that THP generation might be hurt due to fewer
free pages. But probably the extra min_free_kbytes just moves reclaim earlier,
like Michal said.

After removing this automatic min_free_kbytes bump, user can retain the old
behavior by setting a higher min_free_kbytes at boot time.

Best Regards,
Yan, Zi