[RFC PATCH 2/3] mm/compaction: add per-folio isolation tracepoint

[Wandun Chen <chenwandun1@gmail.com>]

From: Wandun Chen <chenwandun@lixiang.com>

Add a tracepoint that fires once per folio successfully isolated by
isolate_migratepages_block(), recording the pfn, isolation mode and
the folio flags. Knowing these makes it easier to debug unexpected
isolation, such as mlocked or unevictable folios showing up on
PREEMPT_RT kernels [1].

Inspired-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
Link: https://lore.kernel.org/all/20250820142919.HTybedrl@linutronix.de/ [1]
---
 include/trace/events/compaction.h | 26 ++++++++++++++++++++++++++
 mm/compaction.c                   |  2 ++
 2 files changed, 28 insertions(+)

diff --git a/include/trace/events/compaction.h b/include/trace/events/compaction.h
index d05759d18538..8b8b3ec0f324 100644
--- a/include/trace/events/compaction.h
+++ b/include/trace/events/compaction.h
@@ -76,6 +76,32 @@ DEFINE_EVENT(mm_compaction_isolate_template, mm_compaction_fast_isolate_freepage
 );
 
 #ifdef CONFIG_COMPACTION
+TRACE_EVENT(mm_compaction_isolate_folio,
+
+	TP_PROTO(unsigned long pfn,
+		isolate_mode_t mode,
+		unsigned long flags),
+
+	TP_ARGS(pfn, mode, flags),
+
+	TP_STRUCT__entry(
+		__field(unsigned long, pfn)
+		__field(isolate_mode_t, mode)
+		__field(unsigned long, flags)
+	),
+
+	TP_fast_assign(
+		__entry->pfn = pfn;
+		__entry->mode = mode;
+		__entry->flags = flags;
+	),
+
+	TP_printk("pfn=0x%lx mode=0x%x flags=%s",
+		__entry->pfn,
+		__entry->mode,
+		show_page_flags(__entry->flags & PAGEFLAGS_MASK))
+);
+
 TRACE_EVENT(mm_compaction_migratepages,
 
 	TP_PROTO(unsigned int nr_migratepages,
diff --git a/mm/compaction.c b/mm/compaction.c
index 7e07b792bcb5..007d5e00a8ae 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1223,6 +1223,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 isolate_success:
 		list_add(&folio->lru, &cc->migratepages);
 isolate_success_no_list:
+		trace_mm_compaction_isolate_folio(folio_pfn(folio), mode,
+						  folio->flags.f);
 		cc->nr_migratepages += folio_nr_pages(folio);
 		nr_isolated += folio_nr_pages(folio);
 		nr_scanned += folio_nr_pages(folio) - 1;
-- 
2.43.0