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

[Matthew Brost <matthew.brost@intel.com>]

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@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 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)
+{
+	/*
+	 * 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.
+	 */
+	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