Memory offline and memory failure handling dissolve free hugetlb folios
and lower nr_hugepages. Nothing records this, so when the pool shrinks
it is hard to tell whether that or a write to nr_hugepages caused it.

Add an htlb_dissolve event, counted for each free folio that
dissolve_free_hugetlb_folio() takes out of the pool.

Signed-off-by: Mark Bundschuh <mkbund@amazon.com>
---
v2: say "takes out of the pool" instead of "frees" (Jiaqi)

v1: https://lore.kernel.org/all/20261105020324.2011790-1-mkbund@amazon.com/

 include/linux/vm_event_item.h | 1 +
 mm/hugetlb.c                  | 1 +
 mm/vmstat.c                   | 1 +
 3 files changed, 3 insertions(+)

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 2628ccda076a..822fe862e5b3 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -71,6 +71,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 #endif
 #ifdef CONFIG_HUGETLB_PAGE
 		HTLB_BUDDY_PGALLOC, HTLB_BUDDY_PGALLOC_FAIL,
+		HTLB_DISSOLVE,
 #endif
 #ifdef CONFIG_CMA
 		CMA_ALLOC_SUCCESS,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index da5d04edf7c1..7c79523b2a3c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2047,6 +2047,7 @@ int dissolve_free_hugetlb_folio(struct folio *folio)
 		}
 
 		update_and_free_hugetlb_folio(h, folio, false);
+		count_vm_event(HTLB_DISSOLVE);
 		return rc;
 	}
 out:
diff --git a/mm/vmstat.c b/mm/vmstat.c
index a3e809c57f29..6282eef7074a 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1397,6 +1397,7 @@ const char * const vmstat_text[] = {
 #ifdef CONFIG_HUGETLB_PAGE
 	[I(HTLB_BUDDY_PGALLOC)]			= "htlb_buddy_alloc_success",
 	[I(HTLB_BUDDY_PGALLOC_FAIL)]		= "htlb_buddy_alloc_fail",
+	[I(HTLB_DISSOLVE)]			= "htlb_dissolve",
 #endif
 #ifdef CONFIG_CMA
 	[I(CMA_ALLOC_SUCCESS)]			= "cma_alloc_success",

base-commit: 05441d697f7f9a927264f4058cbbc6a03dfcac9d
-- 
2.50.1