[PATCH v2] mm: Fix checking unmapped holes for mbind

["Li Xinhai" <lixinhai.lxh@xxxxxxxxx>]

queue_pages_range() will check for unmapped holes besides queue pages for
migration. The rules for checking unmapped holes are:
1 Unmapped holes at any part of the specified range should be reported as
  EFAULT if mbind() for none MPOL_DEFAULT cases;
2 Unmapped holes at any part of the specified range should be ignored if
  mbind() for MPOL_DEFAULT case;
Note that the second rule is the current implementation, but it seems
conflicts the Linux API definition.

queue_pages_test_walk() is fixed by introduce new fields in struct
queue_pages which help to check:
1 holes at head and tail side of specified range;
2 the whole range is in a hole;

Besides, queue_pages_test_walk() must update previous vma record no matter
the current vma should be considered for queue pages or not.

Fixes: 9d8cebd4bcd7 ("mm: fix mbind vma merge problem")
Fixes: 6f4576e3687b ("mempolicy: apply page table walker on queue_pages_range()")
Fixes: 48684a65b4e3 ("mm: pagewalk: fix misbehavior of walk_page_range for vma(VM_PFNMAP)")
Signed-off-by: Li Xinhai <lixinhai.li@xxxxxxxxx>
---
Changes in v2:
  - Fix the unmapped holes checking in queue_pages_test_walk() instead of 
    mbind_rnage().

 mm/mempolicy.c | 44 +++++++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4ae967bcf954..24087dfa4dcd 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -411,6 +411,9 @@ struct queue_pages {
 	unsigned long flags;
 	nodemask_t *nmask;
 	struct vm_area_struct *prev;
+	unsigned long start;
+	unsigned long end;
+	int in_hole;
 };
 
 /*
@@ -618,28 +621,31 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 	unsigned long endvma = vma->vm_end;
 	unsigned long flags = qp->flags;
 
-	/*
-	 * Need check MPOL_MF_STRICT to return -EIO if possible
-	 * regardless of vma_migratable
-	 */
-	if (!vma_migratable(vma) &&
-	    !(flags & MPOL_MF_STRICT))
-		return 1;
-
+	/* range check first */
 	if (endvma > end)
 		endvma = end;
-	if (vma->vm_start > start)
-		start = vma->vm_start;
+	BUG_ON((vma->vm_start > start) || (vma->vm_end < end));
 
+	qp->in_hole = 0;
 	if (!(flags & MPOL_MF_DISCONTIG_OK)) {
-		if (!vma->vm_next && vma->vm_end < end)
+		if ((!vma->vm_next && vma->vm_end < qp->end) ||
+			(vma->vm_next && qp->end < vma->vm_next->vm_start))
 			return -EFAULT;
-		if (qp->prev && qp->prev->vm_end < vma->vm_start)
+		if ((qp->prev && qp->prev->vm_end < vma->vm_start) ||
+			(!qp->prev && qp->start < vma->vm_start))
 			return -EFAULT;
 	}
 
 	qp->prev = vma;
 
+	/*
+	 * Need check MPOL_MF_STRICT to return -EIO if possible
+	 * regardless of vma_migratable
+	 */
+	if (!vma_migratable(vma) &&
+	    !(flags & MPOL_MF_STRICT))
+		return 1;
+
 	if (flags & MPOL_MF_LAZY) {
 		/* Similar to task_numa_work, skip inaccessible VMAs */
 		if (!is_vm_hugetlb_page(vma) &&
@@ -679,14 +685,23 @@ queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
 		nodemask_t *nodes, unsigned long flags,
 		struct list_head *pagelist)
 {
+	int err;
 	struct queue_pages qp = {
 		.pagelist = pagelist,
 		.flags = flags,
 		.nmask = nodes,
 		.prev = NULL,
+		.start = start,
+		.end = end,
+		.in_hole = 1,
 	};
 
-	return walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
+	err = walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
+	/* whole range in unmapped hole */
+	if (qp->in_hole && !(flags & MPOL_MF_DISCONTIG_OK))
+		err = -EFAULT;
+
+	return err;
 }
 
 /*
@@ -738,8 +753,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
 	unsigned long vmend;
 
 	vma = find_vma(mm, start);
-	if (!vma || vma->vm_start > start)
-		return -EFAULT;
+	BUG_ON(!vma);
 
 	prev = vma->vm_prev;
 	if (start > vma->vm_start)
-- 
2.22.0