[RFC v2 16/21] mm: thp: add THP_SPLIT_PMD_FAILED counter

[Usama Arif <usama.arif@linux.dev>]

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                     | 3 +++
 mm/vmstat.c                   | 1 +
 4 files changed, 6 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 55b14ba244b1b..fc0a5e91b4d40 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 2519d579bc1d8..2dae46fff08ae 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2067,8 +2067,10 @@ 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)) {
+					count_vm_event(THP_SPLIT_PMD_FAILED);
 					page_vma_mapped_walk_done(&pvmw);
 					ret = false;
 					break;
@@ -2471,6 +2473,7 @@ 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)) {
+					count_vm_event(THP_SPLIT_PMD_FAILED);
 					page_vma_mapped_walk_done(&pvmw);
 					ret = false;
 					break;
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 667474773dbc7..da276ef0072ed 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1408,6 +1408,7 @@ 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",
-- 
2.47.3