[RFC 2/2] mm: thp: add THP_SPLIT_PMD_PTE_ALLOC_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

Visible via /proc/vmstat as thp_split_pmd_pte_alloc_failed.

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 22a139f82d75f..827c9a8c251de 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -111,6 +111,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		THP_DEFERRED_SPLIT_PAGE,
 		THP_UNDERUSED_SPLIT_PAGE,
 		THP_SPLIT_PMD,
+		THP_SPLIT_PMD_PTE_ALLOC_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 4c9a8d89fc8aa..8d7c9f67f8a1d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3332,6 +3332,7 @@ void __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_PTE_ALLOC_FAILED);
 			mmu_notifier_invalidate_range_end(&range);
 			return;
 		}
diff --git a/mm/rmap.c b/mm/rmap.c
index c6ff23fc12944..5c4afedb29d5a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2070,8 +2070,10 @@ 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_PTE_ALLOC_FAILED);
 					page_vma_mapped_walk_done(&pvmw);
 					ret = false;
 					break;
@@ -2474,6 +2476,7 @@ 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_PTE_ALLOC_FAILED);
 					page_vma_mapped_walk_done(&pvmw);
 					ret = false;
 					break;
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 99270713e0c13..473edfa624a41 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_PTE_ALLOC_FAILED)]	= "thp_split_pmd_pte_alloc_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