On Sun, 8 May 2022 19:47:02 -0700 Sultan Alsawaf sultan@kerneltoast.com wrote:
From: Sultan Alsawaf sultan@kerneltoast.com
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.
--- 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);
Why not simply lock_page() here? The get_page() alone won't protect from all the dire consequences which you have identified?
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);
ditto.
put_page(page);migrate_read_lock(zspage);}- }
- migrate_read_unlock(zspage);
} static int zs_init_fs_context(struct fs_context *fc)