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

[Lorenzo Stoakes (ARM)]

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.
>
> Link: https://lore.kernel.org/r/20260716173504.760369-1-noren@xxxxxxxxxx/
> Suggested-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> Signed-off-by: Nimrod Oren <noren@xxxxxxxxxx>

I do think this is a practical way of preventing runaway increase of reserves.

In the long run we need to rework the pageblock model I think, but this is
a reasonable way forward in the short-medium term.

Obviously I defer to David and the community on this if they think this is
horrible, then let's not.

But I do worry that we'll carry on with unworkably huge reserves on 64 KiB
page size while rejecting every way out until pageblocks are _entirely_
reworked to account for this (probably the long term solution).

So I think this the least-worst approach of those proposed so far for now
:) Therefore:

Acked-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

> ---
> v2:
> * Use a named constant for the cap.
> * Drop explicit linux/sizes.h include.
> * Update min_free_kbytes documentation.
>
> v1:
> * Cap the final recommendation at 1 GiB as suggested by Lorenzo.
> https://lore.kernel.org/r/20260728202014.2517142-1-noren@xxxxxxxxxx/
>
> RFC v1:
> https://lore.kernel.org/r/20260716173504.760369-1-noren@xxxxxxxxxx/
> ---
>  Documentation/admin-guide/sysctl/vm.rst | 8 ++++++++
>  mm/khugepaged.c                         | 7 ++++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 5b318d17aa4b..c0c925e6f504 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -539,6 +539,14 @@ watermark[WMARK_MIN] value for each lowmem zone in the system.
>  Each lowmem zone gets a number of reserved free pages based
>  proportionally on its size.
>
> +When Transparent Hugepage (THP) support is enabled through global or
> +per-size controls, the kernel may raise this value automatically to
> +help keep pageblocks free and reduce fragmentation for THP allocations.
> +The automatic recommendation cannot exceed 5% of low memory or 1 GiB,
> +whichever is lower.  This limit applies only to the automatic
> +recommendation and does not constrain a higher value written by
> +userspace.
> +
>  Some minimal amount of memory is needed to satisfy PF_MEMALLOC
>  allocations; if you set this to lower than 1024KB, your system will
>  become subtly broken, and prone to deadlock under high loads.
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index f49a6710933b..a8fc061d2488 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -101,6 +101,7 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
>
>  static struct kmem_cache *mm_slot_cache __ro_after_init;
>
> +#define THP_MIN_FREE_MAX_PAGES		(SZ_1G / PAGE_SIZE)
>  #define KHUGEPAGED_MIN_MTHP_ORDER	2
>
>  struct collapse_control {
> @@ -3120,9 +3121,13 @@ void set_recommended_min_free_kbytes(void)
>  	recommended_min += pageblock_nr_pages * nr_zones *
>  			   MIGRATE_PCPTYPES * MIGRATE_PCPTYPES;
>
> -	/* don't ever allow to reserve more than 5% of the lowmem */
> +	/*
> +	 * Don't allow the THP recommendation to exceed 5% of lowmem or an
> +	 * absolute cap, whichever is smaller.
> +	 */
>  	recommended_min = min(recommended_min,
>  			      (unsigned long) nr_free_buffer_pages() / 20);
> +	recommended_min = min(recommended_min, THP_MIN_FREE_MAX_PAGES);
>  	recommended_min <<= (PAGE_SHIFT-10);
>
>  	if (recommended_min > min_free_kbytes) {
> --
> 2.45.0
>

--
Cheers, Lorenzo