[RFC PATCH 09/14] mm/khugepaged: record SCAN_PAGE_COMPOUND when scan_pmd() finds THP

["Zach O'Keefe" <zokeefe@google.com>]

When scanning an anon pmd to see if it's eligible for collapse, return
SCAN_PAGE_COMPOUND if the pmd already maps a thp. This is consistent
with handling when scanning file-backed memory.

Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++++++------
 1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ecbd3fc41c80..403578161a3b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1011,6 +1011,38 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 			vm_flags_ignore, vmap);
 }
 
+/*
+ * If returning NULL (meaning the pmd isn't mapped, isn't present, or thp),
+ * write the reason to *result.
+ */
+static pmd_t *find_pmd_or_thp_or_none(struct mm_struct *mm,
+				      unsigned long address,
+				      int *result)
+{
+	pmd_t *pmd = mm_find_pmd_raw(mm, address);
+	pmd_t pmde;
+
+	if (!pmd) {
+		*result = SCAN_PMD_NULL;
+		return NULL;
+	}
+
+	pmde = pmd_read_atomic(pmd);
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	/* See comments in pmd_none_or_trans_huge_or_clear_bad() */
+	barrier();
+#endif
+	if (!pmd_present(pmde) || !pmd_none(pmde)) {
+		*result = SCAN_PMD_NULL;
+		return NULL;
+	} else if (pmd_trans_huge(pmde)) {
+		*result = SCAN_PAGE_COMPOUND;
+		return NULL;
+	}
+	return pmd;
+}
+
 /*
  * Bring missing pages in from swap, to complete THP collapse.
  * Only done if khugepaged_scan_pmd believes it is worthwhile.
@@ -1212,9 +1244,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 		goto out_nolock;
 	}
 
-	pmd = mm_find_pmd(mm, address);
+	pmd = find_pmd_or_thp_or_none(mm, address, &result);
 	if (!pmd) {
-		result = SCAN_PMD_NULL;
 		mmap_read_unlock(mm);
 		goto out_nolock;
 	}
@@ -1287,11 +1318,9 @@ static void scan_pmd(struct mm_struct *mm,
 	mmap_assert_locked(mm);
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
-	pmd = mm_find_pmd(mm, address);
-	if (!pmd) {
-		scan_result->result = SCAN_PMD_NULL;
+	pmd = find_pmd_or_thp_or_none(mm, address, &scan_result->result);
+	if (!pmd)
 		goto out;
-	}
 
 	memset(cc->node_load, 0, sizeof(cc->node_load));
 	pte = pte_offset_map_lock(mm, pmd, address, &ptl);
-- 
2.35.1.616.g0bdcbb4464-goog