On Mon, 29 Jan 2018 18:39:01 -0500 "Zi Yan" <[email protected]> wrote:
> On 29 Jan 2018, at 17:35, Andrew Morton wrote: > > > On Mon, 29 Jan 2018 17:06:14 -0500 "Zi Yan" <[email protected]> wrote: > > > >> I discover that this patch does not hold mmap_sem while migrating pages in > >> do_move_pages_to_node(). > >> > >> A simple fix below moves mmap_sem from add_page_for_migration() > >> to the outmost do_pages_move(): > > > > I'm not surprised. Why does do_move_pages_to_node() need mmap_sem > > and how is a reader to discover that fact??? > > do_move_pages_to_node() calls migrate_pages(), which requires > down_read(&mmap_sem). > > In the outmost do_pages_move(), both add_page_for_migration() and > do_move_pages_to_node() inside it need to hold read lock of mmap_sem. > > Do we need to add comments for both functions? Just for migrate_pages(), I guess. Let's include a description of *why* mmap_sem is needed. What it is protecting.
