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

[Lance Yang]

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 ...