Re: [PATCH v3] mm: remove min_free_kbytes adjustment for THP

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 9/1/26 21:01, 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.
> Commit f000565adb77 ("thp: set recommended min free kbytes") added this
> heuristic to help keep pageblocks free and reduce fragmentation for THP
> allocations.
> 
> The recommendation scales poorly with larger base page sizes. With the
> default arm64 pageblock sizes, the contribution per eligible zone
> before applying the existing cap of 5% of low memory 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
> 
> Even with that cap, min_free_kbytes can reach excessive levels.
> 
> The automatic min_free_kbytes increase predates proactive compaction
> and many subsequent changes to compaction. Given those changes,
> increasing min_free_kbytes for THP by default is no longer clearly
> justified.
> 
> Remove set_recommended_min_free_kbytes() and all associated
> recalculation paths. With this policy gone, min_free_kbytes is
> controlled only by the page allocator's default calculation and the
> vm.min_free_kbytes sysctl. Users who want additional headroom may set a
> higher value via that sysctl.
> 
> Link: https://lore.kernel.org/r/20260831075635.2244437-1-noren@nvidia.com/
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Nimrod Oren <noren@nvidia.com>
> ---
> v3:
> * Replace the 1 GiB cap with removal of the THP-driven min_free_kbytes
>   increase as suggested by Michal.
> * Remove all associated recalculation paths.
> * Make min_free_kbytes, user_min_free_kbytes,
>   calculate_min_free_kbytes(), and setup_per_zone_wmarks() static.
> * Drop the now-unused declarations and khugepaged's page_alloc.h include.
> * Drop the obsolete v2 documentation addition.
> 
> v2:
> * Use a named constant for the cap.
> * Drop explicit linux/sizes.h include.
> * Update min_free_kbytes documentation.
> https://lore.kernel.org/r/20260831075635.2244437-1-noren@nvidia.com/
> 
> v1:
> * Cap the final recommendation at 1 GiB as suggested by Lorenzo.
> https://lore.kernel.org/r/20260728202014.2517142-1-noren@nvidia.com/
> 
> RFC v1:
> https://lore.kernel.org/r/20260716173504.760369-1-noren@nvidia.com/
> ---
>  include/linux/khugepaged.h |  5 ----
>  mm/huge_memory.c           | 14 ---------
>  mm/internal.h              |  8 -----
>  mm/khugepaged.c            | 60 --------------------------------------
>  mm/page_alloc.c            | 11 +++----
>  mm/page_alloc.h            |  2 --
>  mm/shmem.c                 |  7 -----
>  7 files changed, 4 insertions(+), 103 deletions(-)

Given neither of the patchset versions achieved universal acceptance, could
we try just disabling all the automatic (re)calculations of min_free_kbytes
the first moment the sysctl gets adjusted from userspace?