[PATCH v4 01/13] mm/khugepaged: record SCAN_PMD_MAPPED 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_PMD_MAPPED if the pmd already maps a THP.  Note that
SCAN_PMD_MAPPED is different from SCAN_PAGE_COMPOUND used in the
file-collapse path, since the latter might identify pte-mapped compound
pages.  This is required by MADV_COLLAPSE which necessarily needs to
know what hugepage-aligned/sized regions are already pmd-mapped.

Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 include/trace/events/huge_memory.h |  1 +
 mm/internal.h                      |  1 +
 mm/khugepaged.c                    | 30 ++++++++++++++++++++++++++----
 mm/rmap.c                          | 15 +++++++++++++--
 4 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index d651f3437367..55392bf30a03 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -11,6 +11,7 @@
 	EM( SCAN_FAIL,			"failed")			\
 	EM( SCAN_SUCCEED,		"succeeded")			\
 	EM( SCAN_PMD_NULL,		"pmd_null")			\
+	EM( SCAN_PMD_MAPPED,		"page_pmd_mapped")		\
 	EM( SCAN_EXCEED_NONE_PTE,	"exceed_none_pte")		\
 	EM( SCAN_EXCEED_SWAP_PTE,	"exceed_swap_pte")		\
 	EM( SCAN_EXCEED_SHARED_PTE,	"exceed_shared_pte")		\
diff --git a/mm/internal.h b/mm/internal.h
index 0667abd57634..51ae9f71a2a3 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -172,6 +172,7 @@ extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason
 /*
  * in mm/rmap.c:
  */
+pmd_t *mm_find_pmd_raw(struct mm_struct *mm, unsigned long address);
 extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
 
 /*
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index eb444fd45568..2c2ed6b4d96c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -28,6 +28,7 @@ enum scan_result {
 	SCAN_FAIL,
 	SCAN_SUCCEED,
 	SCAN_PMD_NULL,
+	SCAN_PMD_MAPPED,
 	SCAN_EXCEED_NONE_PTE,
 	SCAN_EXCEED_SWAP_PTE,
 	SCAN_EXCEED_SHARED_PTE,
@@ -977,6 +978,29 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 	return 0;
 }
 
+static int find_pmd_or_thp_or_none(struct mm_struct *mm,
+				   unsigned long address,
+				   pmd_t **pmd)
+{
+	pmd_t pmde;
+
+	*pmd = mm_find_pmd_raw(mm, address);
+	if (!*pmd)
+		return SCAN_PMD_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))
+		return SCAN_PMD_NULL;
+	if (pmd_trans_huge(pmde))
+		return SCAN_PMD_MAPPED;
+	return SCAN_SUCCEED;
+}
+
 /*
  * Bring missing pages in from swap, to complete THP collapse.
  * Only done if khugepaged_scan_pmd believes it is worthwhile.
@@ -1228,11 +1252,9 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
-	pmd = mm_find_pmd(mm, address);
-	if (!pmd) {
-		result = SCAN_PMD_NULL;
+	result = find_pmd_or_thp_or_none(mm, address, &pmd);
+	if (result != SCAN_SUCCEED)
 		goto out;
-	}
 
 	memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
 	pte = pte_offset_map_lock(mm, pmd, address, &ptl);
diff --git a/mm/rmap.c b/mm/rmap.c
index 94d6b24a1ac2..6980b4011bf8 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -759,13 +759,12 @@ unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
 	return vma_address(page, vma);
 }
 
-pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
+pmd_t *mm_find_pmd_raw(struct mm_struct *mm, unsigned long address)
 {
 	pgd_t *pgd;
 	p4d_t *p4d;
 	pud_t *pud;
 	pmd_t *pmd = NULL;
-	pmd_t pmde;
 
 	pgd = pgd_offset(mm, address);
 	if (!pgd_present(*pgd))
@@ -780,6 +779,18 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
 		goto out;
 
 	pmd = pmd_offset(pud, address);
+out:
+	return pmd;
+}
+
+pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
+{
+	pmd_t pmde;
+	pmd_t *pmd;
+
+	pmd = mm_find_pmd_raw(mm, address);
+	if (!pmd)
+		goto out;
 	/*
 	 * Some THP functions use the sequence pmdp_huge_clear_flush(), set_pmd_at()
 	 * without holding anon_vma lock for write.  So when looking for a
-- 
2.36.0.464.gb9c8b46e94-goog