[PATCH v2 4/7] mm/khugepaged: fix outdated comments

[Nico Pache <npache@redhat.com>]

Fix comment in collapse_scan_pmd() that still described the old
folio_mapcount() > folio_ref_count() check and a "512" false-positive
scenario. The code now uses folio_expected_ref_count() != folio_ref_count()
which doesn't suffer from the same limitation.

Fix comment in collapse_huge_page() that referenced ptep_clear_flush,
when the code actually uses pmdp_collapse_flush.

Fix comment in __collapse_huge_page_swapin() that referenced the old
function name khugepaged_scan_pmd, now collapse_scan_pmd.

Also clean up some simple typos and stale terminology (mmap_sem ->
mmap_lock, PG_lock -> folio lock, page -> folio, grammar).

We also clarify a comment regarding where the max_ptes_none check is
deferred to in mthp_collapse() from the original collapse_scan_pmd check.

Acked-by: Usama Arif <usama.arif@linux.dev>
Assisted-by: Cursor(claude-sonnet-4):4.6
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f65bbe2051b3..d785468ffb96 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -620,7 +620,7 @@ void __khugepaged_exit(struct mm_struct *mm)
 		/*
 		 * This is required to serialize against
 		 * collapse_test_exit() (which is guaranteed to run
-		 * under mmap sem read mode). Stop here (after we return all
+		 * under mmap_lock read mode). Stop here (after we return all
 		 * pagetables will be destroyed) until khugepaged has finished
 		 * working on the pagetables under the mmap_lock.
 		 */
@@ -789,7 +789,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		/*
 		 * We can do it before folio_isolate_lru because the
-		 * folio can't be freed from under us. NOTE: PG_lock
+		 * folio can't be freed from under us. NOTE: folio lock
 		 * is needed to serialize against split_huge_page
 		 * when invoked from the VM.
 		 */
@@ -816,7 +816,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		}
 
 		/*
-		 * Isolate the page to avoid collapsing an hugepage
+		 * Isolate the folio to avoid collapsing a hugepage
 		 * currently in use by the VM.
 		 */
 		if (!folio_isolate_lru(folio)) {
@@ -1101,7 +1101,7 @@ static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned l
 		return SCAN_VMA_CHECK;
 	/*
 	 * Anon VMA expected, the address may be unmapped then
-	 * remapped to file after khugepaged reaquired the mmap_lock.
+	 * remapped to file after khugepaged reacquired the mmap_lock.
 	 *
 	 * thp_vma_allowable_orders may return true for qualified file
 	 * vmas.
@@ -1159,7 +1159,7 @@ static enum scan_result check_pmd_still_valid(struct mm_struct *mm,
 
 /*
  * Bring missing pages in from swap, to complete THP collapse.
- * Only done if khugepaged_scan_pmd believes it is worthwhile.
+ * Only done if collapse_scan_pmd believes it is worthwhile.
  *
  * For mTHP orders the function bails on the first swap entry, because
  * faulting pages back in during collapse could re-populate PTEs that
@@ -1351,8 +1351,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 	mmap_read_unlock(mm);
 	/*
 	 * Prevent all access to pagetables with the exception of
-	 * gup_fast later handled by the ptep_clear_flush and the VM
-	 * handled by the anon_vma lock + PG_lock.
+	 * gup_fast later handled by the pmdp_collapse_flush and the VM
+	 * handled by the anon_vma lock + folio lock.
 	 *
 	 * UFFDIO_MOVE is prevented to race as well thanks to the
 	 * mmap_lock.
@@ -1643,7 +1643,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 
 	/*
 	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
-	 * scan all pages to populate the bitmap for mTHP collapse.
+	 * scan all pages to populate the bitmap for mTHP collapse. The bitmap
+	 * is then checked again in mthp_collapse() for each attempted order.
 	 */
 	if (enabled_orders != BIT(HPAGE_PMD_ORDER))
 		max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
@@ -1764,12 +1765,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		/*
 		 * Check if the page has any GUP (or other external) pins.
 		 *
-		 * Here the check may be racy:
-		 * it may see folio_mapcount() > folio_ref_count().
-		 * But such case is ephemeral we could always retry collapse
-		 * later.  However it may report false positive if the page
-		 * has excessive GUP pins (i.e. 512).  Anyway the same check
-		 * will be done again later the risk seems low.
+		 * Here the check is racy, but such case is ephemeral and
+		 * we could always retry collapse later. Anyway the same
+		 * check will be done again later the risk seems low.
 		 */
 		if (folio_expected_ref_count(folio) != folio_ref_count(folio)) {
 			result = SCAN_PAGE_COUNT;
-- 
2.54.0