[PATCH] fix mremap bug of failing to split thp

[Naoya Horiguchi]

The patch "thp: optimize away unnecessary page table locking" introduced
a bug to move_page_tables(), where we fail to split thp when move_huge_pmd()
is not called. To fix it, this patch reverts the return value changes and
readd if (!err) block.

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
---
 fs/proc/task_mmu.c |    6 +++---
 mm/huge_memory.c   |   13 ++++++-------
 mm/mremap.c        |    2 ++
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 7810281..2d12325 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -394,7 +394,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	pte_t *pte;
 	spinlock_t *ptl;
 
-	if (pmd_trans_huge_lock(pmd, vma)) {
+	if (pmd_trans_huge_lock(pmd, vma) == 1) {
 		smaps_pte_entry(pmd_to_pte_t(pmd), addr, HPAGE_PMD_SIZE, walk);
 		spin_unlock(&walk->mm->page_table_lock);
 		mss->anonymous_thp += HPAGE_PMD_SIZE;
@@ -696,7 +696,7 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	/* find the first VMA at or above 'addr' */
 	vma = find_vma(walk->mm, addr);
 
-	if (pmd_trans_huge_lock(pmd, vma)) {
+	if (pmd_trans_huge_lock(pmd, vma) == 1) {
 		for (; addr != end; addr += PAGE_SIZE) {
 			unsigned long offset = (addr & ~PAGEMAP_WALK_MASK)
 				>> PAGE_SHIFT;
@@ -973,7 +973,7 @@ static int gather_pte_stats(pmd_t *pmd, unsigned long addr,
 
 	md = walk->private;
 
-	if (pmd_trans_huge_lock(pmd, md->vma)) {
+	if (pmd_trans_huge_lock(pmd, md->vma) == 1) {
 		pte_t huge_pte = pmd_to_pte_t(pmd);
 		struct page *page;
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bbf57b5..f342bb2 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1030,7 +1030,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 {
 	int ret = 0;
 
-	if (__pmd_trans_huge_lock(pmd, vma)) {
+	if (__pmd_trans_huge_lock(pmd, vma) == 1) {
 		struct page *page;
 		pgtable_t pgtable;
 		pgtable = get_pmd_huge_pte(tlb->mm);
@@ -1056,7 +1056,7 @@ int mincore_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 {
 	int ret = 0;
 
-	if (__pmd_trans_huge_lock(pmd, vma)) {
+	if (__pmd_trans_huge_lock(pmd, vma) == 1) {
 		/*
 		 * All logical pages in the range are present
 		 * if backed by a huge page.
@@ -1094,12 +1094,11 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
 		goto out;
 	}
 
-	if (__pmd_trans_huge_lock(old_pmd, vma)) {
+	if ((ret = __pmd_trans_huge_lock(old_pmd, vma)) == 1) {
 		pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
 		VM_BUG_ON(!pmd_none(*new_pmd));
 		set_pmd_at(mm, new_addr, new_pmd, pmd);
 		spin_unlock(&mm->page_table_lock);
-		ret = 1;
 	}
 out:
 	return ret;
@@ -1111,7 +1110,7 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	struct mm_struct *mm = vma->vm_mm;
 	int ret = 0;
 
-	if (__pmd_trans_huge_lock(pmd, vma)) {
+	if (__pmd_trans_huge_lock(pmd, vma) == 1) {
 		pmd_t entry;
 		entry = pmdp_get_and_clear(mm, addr, pmd);
 		entry = pmd_modify(entry, newprot);
@@ -1125,7 +1124,7 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 
 /*
  * Returns 1 if a given pmd maps a stable (not under splitting) thp.
- * Returns 0 otherwise.
+ * Returns -1 if it maps a thp under splitting. Returns 0 otherwise.
  *
  * Note that if it returns 1, this routine returns without unlocking page
  * table locks. So callers must unlock them.
@@ -1137,7 +1136,7 @@ int __pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
 		if (unlikely(pmd_trans_splitting(*pmd))) {
 			spin_unlock(&vma->vm_mm->page_table_lock);
 			wait_split_huge_page(vma->anon_vma, pmd);
-			return 0;
+			return -1;
 		} else {
 			/* Thp mapped by 'pmd' is stable, so we can
 			 * handle it as it is. */
diff --git a/mm/mremap.c b/mm/mremap.c
index 22458b9..87bb839 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -155,6 +155,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 			if (err > 0) {
 				need_flush = true;
 				continue;
+			} else if (!err) {
+				split_huge_page_pmd(vma->vm_mm, old_pmd);
 			}
 			VM_BUG_ON(pmd_trans_huge(*old_pmd));
 		}
-- 
1.7.7.6

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