Re: [PATCH v2 1/5] mm: Introduce zone_appears_fragmented()

[Andi Shyti]

Hi Matt,

On Wed, Apr 22, 2026 at 10:56:52PM -0700, Matthew Brost wrote:
> Introduce zone_appears_fragmented() as a lightweight helper to allow
> subsystems to make coarse decisions about reclaim behavior in the
> presence of likely fragmentation.
> 
> The helper implements a simple heuristic: if the number of free pages
> in a zone exceeds twice the high watermark, the zone is considered to
> have ample free memory and allocation failures are more likely due to
> fragmentation than overall memory pressure.
> 
> This is intentionally imprecise and is not meant to replace the core
> MM compaction or fragmentation accounting logic. Instead, it provides
> a cheap signal for callers (e.g., shrinkers) that wish to avoid
> overly aggressive reclaim when sufficient free memory exists but
> high-order allocations may still fail.
> 
> No functional changes; this is a preparatory helper for future users.
> 
> Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> ---
>  include/linux/vmstat.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 3c9c266cf782..568d9f4f1a1f 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -483,6 +483,19 @@ static inline const char *zone_stat_name(enum zone_stat_item item)
>  	return vmstat_text[item];
>  }
>  
> +static inline bool zone_appears_fragmented(struct zone *zone)

this is a bit of a strong statement and the function name might
be misleading. You received some suggestions from David and I
would rename this function to something like
"zone_maybe_fragmented()".

> +{
> +	/*
> +	 * Simple heuristic: if the number of free pages is more than twice the
> +	 * high watermark, this strongly suggests that the zone is heavily
> +	 * fragmented when called from a shrinker.
> +	 */

The commit log explains it a bit better. The heuristic statement
here is too strong and it still sounds stronger than it should.

Andi

> +	if (zone_page_state(zone, NR_FREE_PAGES) > high_wmark_pages(zone) * 2)
> +		return true;
> +
> +	return false;
> +}
> +
>  #ifdef CONFIG_NUMA
>  static inline const char *numa_stat_name(enum numa_stat_item item)
>  {
> -- 
> 2.34.1
>