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

[Zi Yan]

On 31 Aug 2026, at 5:00, Lance Yang wrote:

> On 2026/8/31 15:56, 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.
>
> Sorry, I haven't been following the previous discussion.
>
> One nit: why 1 GiB? It feels a bit arbitrary to me ...

It is. Just better than 5.5GiB in the existing configuration.

Best Regards,
Yan, Zi