[PATCH] mm: move_pages: return valid node id in status if the page is already on the target node

[Yang Shi]

Felix Abecassis reports move_pages() would return random status if the
pages are already on the target node by the below test program:

---8<---

int main(void)
{
	const long node_id = 1;
	const long page_size = sysconf(_SC_PAGESIZE);
	const int64_t num_pages = 8;

	unsigned long nodemask =  1 << node_id;
	long ret = set_mempolicy(MPOL_BIND, &nodemask, sizeof(nodemask));
	if (ret < 0)
		return (EXIT_FAILURE);

	void **pages = malloc(sizeof(void*) * num_pages);
	for (int i = 0; i < num_pages; ++i) {
		pages[i] = mmap(NULL, page_size, PROT_WRITE | PROT_READ,
				MAP_PRIVATE | MAP_POPULATE | MAP_ANONYMOUS,
				-1, 0);
		if (pages[i] == MAP_FAILED)
			return (EXIT_FAILURE);
	}

	ret = set_mempolicy(MPOL_DEFAULT, NULL, 0);
	if (ret < 0)
		return (EXIT_FAILURE);

	int *nodes = malloc(sizeof(int) * num_pages);
	int *status = malloc(sizeof(int) * num_pages);
	for (int i = 0; i < num_pages; ++i) {
		nodes[i] = node_id;
		status[i] = 0xd0; /* simulate garbage values */
	}

	ret = move_pages(0, num_pages, pages, nodes, status, MPOL_MF_MOVE);
	printf("move_pages: %ld\n", ret);
	for (int i = 0; i < num_pages; ++i)
		printf("status[%d] = %d\n", i, status[i]);
}
---8<---

Then running the program would return nonsense status values:
$ ./move_pages_bug
move_pages: 0
status[0] = 208
status[1] = 208
status[2] = 208
status[3] = 208
status[4] = 208
status[5] = 208
status[6] = 208
status[7] = 208

This is because the status is not set if the page is already on the
target node, but move_pages() should return valid status as long as it
succeeds.  The valid status may be errno or node id.

We can't simply initialize status array to zero since the pages may be
not on node 0.  Fix it by updating status with node id which the page is
already on.  And, it looks we have to update the status inside
add_page_for_migration() since the page struct is not available outside
it.

Make add_page_for_migration() return 1 if store_status() is failed in
order to not mix up the status value since -EFAULT is also a valid
status.

Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
Reported-by: Felix Abecassis <fabecassis@xxxxxxxxxx>
Tested-by: Felix Abecassis <fabecassis@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> 4.17+
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
John found another return value inconsistency between the implementation and
the manpage.  The manpage says it should return -ENOENT if the page is already
on the target node, but it doesn't.  It looks the original code didn't return
-ENOENT either, I'm not sure if this is a document issue or not.  Anyway this
is another issue, once we confirm it we can fix it later.
  
 mm/migrate.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index a8f87cb..c427bb2 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1512,17 +1512,21 @@ static int do_move_pages_to_node(struct mm_struct *mm,
 /*
  * Resolves the given address to a struct page, isolates it from the LRU and
  * puts it to the given pagelist.
- * Returns -errno if the page cannot be found/isolated or 0 when it has been
- * queued or the page doesn't need to be migrated because it is already on
- * the target node
+ * Returns:
+ *     errno - if the page cannot be found/isolated
+ *     0 - when it has been queued or the page doesn't need to be migrated
+ *         because it is already on the target node
+ *     1 - if store_status() is failed
  */
 static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
-		int node, struct list_head *pagelist, bool migrate_all)
+		int node, struct list_head *pagelist, bool migrate_all,
+		int __user *status, int start)
 {
 	struct vm_area_struct *vma;
 	struct page *page;
 	unsigned int follflags;
 	int err;
+	bool same_node = false;
 
 	down_read(&mm->mmap_sem);
 	err = -EFAULT;
@@ -1543,8 +1547,10 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
 		goto out;
 
 	err = 0;
-	if (page_to_nid(page) == node)
+	if (page_to_nid(page) == node) {
+		same_node = true;
 		goto out_putpage;
+	}
 
 	err = -EACCES;
 	if (page_mapcount(page) > 1 && !migrate_all)
@@ -1578,6 +1584,16 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
 	put_page(page);
 out:
 	up_read(&mm->mmap_sem);
+
+	/*
+	 * Must call store_status() after releasing mmap_sem since put_user
+	 * need acquire mmap_sem too, otherwise potential deadlock may exist.
+	 */
+	if (same_node) {
+		if (store_status(status, start, node, 1))
+			err = 1;
+	}
+
 	return err;
 }
 
@@ -1639,10 +1655,16 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 		 * report them via status
 		 */
 		err = add_page_for_migration(mm, addr, current_node,
-				&pagelist, flags & MPOL_MF_MOVE_ALL);
+				&pagelist, flags & MPOL_MF_MOVE_ALL, status,
+				i);
+
 		if (!err)
 			continue;
 
+		/* store_status() failed in add_page_for_migration() */
+		if (err > 0)
+			goto out_flush;
+
 		err = store_status(status, i, err, 1);
 		if (err)
 			goto out_flush;
-- 
1.8.3.1