mm/vmstat: add events for THP max_ptes_* exceeds

There are interfaces to adjust max_ptes_none, max_ptes_swap,
max_ptes_shared values, see
  /sys/kernel/mm/transparent_hugepage/khugepaged/.

But system administrator may not know which value is the best.  So Add
those events to support adjusting max_ptes_* to suitable values.

For example, if default max_ptes_swap value causes too much failures,
and system uses zram whose IO is fast, administrator could increase
max_ptes_swap until THP_SCAN_EXCEED_SWAP_PTE not increase anymore.

Link: https://lkml.kernel.org/r/20211225094036.574157-1-yang.yang29@zte.com.cn
Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Saravanan D <saravanand@fb.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: yangyang20220519

include/linux/vm_event_item.h

@@ -98,6 +98,9 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		THP_SPLIT_PAGE_FAILED,
 		THP_DEFERRED_SPLIT_PAGE,
 		THP_SPLIT_PMD,
+		THP_SCAN_EXCEED_NONE_PTE,
+		THP_SCAN_EXCEED_SWAP_PTE,
+		THP_SCAN_EXCEED_SHARED_PTE,
 #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 		THP_SPLIT_PUD,
 #endif

mm/khugepaged.c

@@ -618,6 +618,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
+				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 				goto out;
 			}
 		}
@@ -636,6 +637,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		if (page_mapcount(page) > 1 &&
 				++shared > khugepaged_max_ptes_shared) {
 			result = SCAN_EXCEED_SHARED_PTE;
+			count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 			goto out;
 		}
 
@@ -1253,6 +1255,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 				continue;
 			} else {
 				result = SCAN_EXCEED_SWAP_PTE;
+				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				goto out_unmap;
 			}
 		}
@@ -1262,6 +1265,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
+				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 				goto out_unmap;
 			}
 		}
@@ -1290,6 +1294,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 		if (page_mapcount(page) > 1 &&
 				++shared > khugepaged_max_ptes_shared) {
 			result = SCAN_EXCEED_SHARED_PTE;
+			count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 			goto out_unmap;
 		}
 
@@ -2000,6 +2005,7 @@ static void khugepaged_scan_file(struct mm_struct *mm,
 		if (xa_is_value(page)) {
 			if (++swap > khugepaged_max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
+				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				break;
 			}
 			continue;
@@ -2046,6 +2052,7 @@ static void khugepaged_scan_file(struct mm_struct *mm,
 	if (result == SCAN_SUCCEED) {
 		if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
 			result = SCAN_EXCEED_NONE_PTE;
+			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 		} else {
 			node = khugepaged_find_target_node();
 			collapse_file(mm, file, start, hpage, node);

mm/vmstat.c

@@ -1353,6 +1353,9 @@ const char * const vmstat_text[] = {
 	"thp_split_page_failed",
 	"thp_deferred_split_page",
 	"thp_split_pmd",
+	"thp_scan_exceed_none_pte",
+	"thp_scan_exceed_swap_pte",
+	"thp_scan_exceed_share_pte",
 #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 	"thp_split_pud",
 #endif