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

[Kiryl Shutsemau]

On Mon, Aug 31, 2026 at 10:56:35AM +0300, 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 am not against an upper limit for min_free_kbytes, but I think the
root cause of the problem you see is the size of the page block.

We at Meta switched to 2M page blocks on 64k ARMs recently, targeting 2M
mTHPs. A 512M pageblock is not practical.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov