mm: mempolicy: keep VMA walk if both MPOL_MF_STRICT and MPOL_MF_MOVE are specified

When calling mbind() with MPOL_MF_{MOVE|MOVEALL} | MPOL_MF_STRICT, kernel
should attempt to migrate all existing pages, and return -EIO if there is
misplaced or unmovable page. Then commit 6f4576e3687b ("mempolicy: apply
page table walker on queue_pages_range()") messed up the return value and
didn't break VMA scan early ianymore when MPOL_MF_STRICT alone. The
return value problem was fixed by commit a7f40cfe3b7a ("mm: mempolicy:
make mbind() return -EIO when MPOL_MF_STRICT is specified"), but it broke
the VMA walk early if unmovable page is met, it may cause some pages are
not migrated as expected.

The code should conceptually do:

if (MPOL_MF_MOVE|MOVEALL)
scan all vmas
try to migrate the existing pages
return success
else if (MPOL_MF_MOVE* | MPOL_MF_STRICT)
scan all vmas
try to migrate the existing pages
return -EIO if unmovable or migration failed
else /* MPOL_MF_STRICT alone */
break early if meets unmovable and don't call mbind_range() at all
else /* none of those flags */
check the ranges in test_walk, EFAULT without mbind_range() if discontig.

Fixed the behavior.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when MPOL_MF_STRICT is specified")
Signed-off-by: Yang Shi <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Oscar Salvador <[e-mail]>
Cc: Rafael Aquini <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: <[e-mail]> [4.9+]
Signed-off-by: Andrew Morton <[e-mail]>

mm/mempolicy.c

···
 	unsigned long start;
 	unsigned long end;
 	struct vm_area_struct *first;
+	bool has_unmovable;
 };
 
 /*
···
 /*
  * queue_folios_pmd() has three possible return values:
  * 0 - folios are placed on the right node or queued successfully, or
- *     special page is met, i.e. huge zero page.
- * 1 - there is unmovable folio, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
- *     specified.
+ *     special page is met, i.e. zero page, or unmovable page is found
+ *     but continue walking (indicated by queue_pages.has_unmovable).
  * -EIO - is migration entry or only MPOL_MF_STRICT was specified and an
  *        existing folio was already on a node that does not follow the
  *        policy.
···
 	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
 		if (!vma_migratable(walk->vma) ||
 		    migrate_folio_add(folio, qp->pagelist, flags)) {
-			ret = 1;
+			qp->has_unmovable = true;
 			goto unlock;
 		}
 	} else
···
  *
  * queue_folios_pte_range() has three possible return values:
  * 0 - folios are placed on the right node or queued successfully, or
- *     special page is met, i.e. zero page.
- * 1 - there is unmovable folio, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
- *     specified.
+ *     special page is met, i.e. zero page, or unmovable page is found
+ *     but continue walking (indicated by queue_pages.has_unmovable).
  * -EIO - only MPOL_MF_STRICT was specified and an existing folio was already
  *        on a node that does not follow the policy.
  */
···
 	struct folio *folio;
 	struct queue_pages *qp = walk->private;
 	unsigned long flags = qp->flags;
-	bool has_unmovable = false;
 	pte_t *pte, *mapped_pte;
 	pte_t ptent;
 	spinlock_t *ptl;
···
 		if (!queue_folio_required(folio, qp))
 			continue;
 		if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
-			/* MPOL_MF_STRICT must be specified if we get here */
-			if (!vma_migratable(vma)) {
-				has_unmovable = true;
-				break;
-			}
+			/*
+			 * MPOL_MF_STRICT must be specified if we get here.
+			 * Continue walking vmas due to MPOL_MF_MOVE* flags.
+			 */
+			if (!vma_migratable(vma))
+				qp->has_unmovable = true;
 
 			/*
 			 * Do not abort immediately since there may be
···
 			 * need migrate other LRU pages.
 			 */
 			if (migrate_folio_add(folio, qp->pagelist, flags))
-				has_unmovable = true;
+				qp->has_unmovable = true;
 		} else
 			break;
 	}
 	pte_unmap_unlock(mapped_pte, ptl);
 	cond_resched();
-
-	if (has_unmovable)
-		return 1;
 
 	return addr != end ? -EIO : 0;
 }
···
 		 * Detecting misplaced folio but allow migrating folios which
 		 * have been queued.
 		 */
-		ret = 1;
+		qp->has_unmovable = true;
 		goto unlock;
 	}
 
···
 			 * Failed to isolate folio but allow migrating pages
 			 * which have been queued.
 			 */
-			ret = 1;
+			qp->has_unmovable = true;
 	}
 unlock:
 	spin_unlock(ptl);
···
 		.start = start,
 		.end = end,
 		.first = NULL,
+		.has_unmovable = false,
 	};
 	const struct mm_walk_ops *ops = lock_vma ?
 			&queue_pages_lock_vma_walk_ops : &queue_pages_walk_ops;
 
 	err = walk_page_range(mm, start, end, ops, &qp);
 
+	if (qp.has_unmovable)
+		err = 1;
 	if (!qp.first)
 		/* whole range in hole */
 		err = -EFAULT;
···
 				putback_movable_pages(&pagelist);
 		}
 
-		if ((ret > 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
+		if (((ret > 0) || nr_failed) && (flags & MPOL_MF_STRICT))
 			err = -EIO;
 	} else {
 up_out: