[PATCH] zsmalloc: Fix races between asynchronous zspage free and page migration

[Sultan Alsawaf]

From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>

The asynchronous zspage free worker tries to lock a zspage's entire page
list without defending against page migration. Since pages which haven't
yet been locked can concurrently migrate off the zspage page list while
lock_zspage() churns away, lock_zspage() can suffer from a few different
lethal races. It can lock a page which no longer belongs to the zspage and
unsafely dereference page_private(), it can unsafely dereference a torn
pointer to the next page (since there's a data race), and it can observe a
spurious NULL pointer to the next page and thus not lock all of the
zspage's pages (since a single page migration will reconstruct the entire
page list, and create_page_chain() unconditionally zeroes out each list
pointer in the process).

Fix the races by using migrate_read_lock() in lock_zspage() to synchronize
with page migration.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 48b4800a1c6a ("zsmalloc: page migration support")
Signed-off-by: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
---
 mm/zsmalloc.c | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 9152fbde33b5..5d5fc04385b8 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1718,11 +1718,40 @@ static enum fullness_group putback_zspage(struct size_class *class,
  */
 static void lock_zspage(struct zspage *zspage)
 {
-	struct page *page = get_first_page(zspage);
+	struct page *curr_page, *page;
 
-	do {
-		lock_page(page);
-	} while ((page = get_next_page(page)) != NULL);
+	/*
+	 * Pages we haven't locked yet can be migrated off the list while we're
+	 * trying to lock them, so we need to be careful and only attempt to
+	 * lock each page under migrate_read_lock(). Otherwise, the page we lock
+	 * may no longer belong to the zspage. This means that we may wait for
+	 * the wrong page to unlock, so we must take a reference to the page
+	 * prior to waiting for it to unlock outside migrate_read_lock().
+	 */
+	while (1) {
+		migrate_read_lock(zspage);
+		page = get_first_page(zspage);
+		if (trylock_page(page))
+			break;
+		get_page(page);
+		migrate_read_unlock(zspage);
+		wait_on_page_locked(page);
+		put_page(page);
+	}
+
+	curr_page = page;
+	while ((page = get_next_page(curr_page))) {
+		if (trylock_page(page)) {
+			curr_page = page;
+		} else {
+			get_page(page);
+			migrate_read_unlock(zspage);
+			wait_on_page_locked(page);
+			put_page(page);
+			migrate_read_lock(zspage);
+		}
+	}
+	migrate_read_unlock(zspage);
 }
 
 static int zs_init_fs_context(struct fs_context *fc)
-- 
2.36.0