Add a vmstat counter to track PTE allocation failures during PMD split.
This enables monitoring of split failures due to memory pressure after
the lazy PTE page table allocation change.

The counter is incremented in three places:
- __split_huge_pmd(): Main entry point for splitting a PMD
- try_to_unmap_one(): When reclaim needs to split a PMD-mapped THP
- try_to_migrate_one(): When migration needs to split a PMD-mapped THP

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 include/linux/vm_event_item.h | 1 +
 mm/huge_memory.c              | 1 +
 mm/rmap.c                     | 7 +++++++
 mm/vmstat.c                   | 1 +
 4 files changed, 10 insertions(+)

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 03fe95f5a0201..ce696cf7d6321 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -98,6 +98,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		THP_DEFERRED_SPLIT_PAGE,
 		THP_UNDERUSED_SPLIT_PAGE,
 		THP_SPLIT_PMD,
+		THP_SPLIT_PMD_FAILED,
 		THP_SCAN_EXCEED_NONE_PTE,
 		THP_SCAN_EXCEED_SWAP_PTE,
 		THP_SCAN_EXCEED_SHARED_PTE,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 48c4884a6f386..b93718931e729 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3347,6 +3347,7 @@ int __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	if (vma_is_anonymous(vma) && !arch_needs_pgtable_deposit()) {
 		pgtable = pte_alloc_one(vma->vm_mm);
 		if (!pgtable) {
+			count_vm_event(THP_SPLIT_PMD_FAILED);
 			mmu_notifier_invalidate_range_end(&range);
 			return -ENOMEM;
 		}
diff --git a/mm/rmap.c b/mm/rmap.c
index efbcdd3b32632..a0180f62d9f69 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2087,8 +2087,12 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				pgtable_t pgtable = prealloc_pte;
 
 				prealloc_pte = NULL;
+
 				if (!arch_needs_pgtable_deposit() && !pgtable &&
 				    vma_is_anonymous(vma)) {
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE)
+					count_vm_event(THP_SPLIT_PMD_FAILED);
+#endif
 					page_vma_mapped_walk_done(&pvmw);
 					ret = false;
 					break;
@@ -2491,6 +2495,9 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				prealloc_pte = NULL;
 				if (!arch_needs_pgtable_deposit() && !pgtable &&
 				    vma_is_anonymous(vma)) {
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE)
+					count_vm_event(THP_SPLIT_PMD_FAILED);
+#endif
 					page_vma_mapped_walk_done(&pvmw);
 					ret = false;
 					break;
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 2370c6fb1fcd6..b8df9c7296d8a 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1409,6 +1409,7 @@ const char * const vmstat_text[] = {
 	[I(THP_DEFERRED_SPLIT_PAGE)]		= "thp_deferred_split_page",
 	[I(THP_UNDERUSED_SPLIT_PAGE)]		= "thp_underused_split_page",
 	[I(THP_SPLIT_PMD)]			= "thp_split_pmd",
+	[I(THP_SPLIT_PMD_FAILED)]		= "thp_split_pmd_failed",
 	[I(THP_SCAN_EXCEED_NONE_PTE)]		= "thp_scan_exceed_none_pte",
 	[I(THP_SCAN_EXCEED_SWAP_PTE)]		= "thp_scan_exceed_swap_pte",
 	[I(THP_SCAN_EXCEED_SHARED_PTE)]		= "thp_scan_exceed_share_pte",
-- 
2.52.0