From dc386d4d1e98bb39fb967ee156cd456c802fc692 Mon Sep 17 00:00:00 2001 From: KAMEZAWA Hiroyuki Date: Thu, 26 Jul 2007 10:41:07 -0700 Subject: memory unplug: migration by kernel In usual, migrate_pages(page,,) is called with holding mm->sem by system call. (mm here is a mm_struct which maps the migration target page.) This semaphore helps avoiding some race conditions. But, if we want to migrate a page by some kernel codes, we have to avoid some races. This patch adds check code for following race condition. 1. A page which page->mapping==NULL can be target of migration. Then, we have to check page->mapping before calling try_to_unmap(). 2. anon_vma can be freed while page is unmapped, but page->mapping remains as it was. We drop page->mapcount to be 0. Then we cannot trust page->mapping. So, use rcu_read_lock() to prevent anon_vma pointed by page->mapping from being freed during migration. Signed-off-by: KAMEZAWA Hiroyuki Acked-by: Christoph Lameter Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/migrate.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/mm/migrate.c b/mm/migrate.c index 34d8ada053e4..c8d87221f368 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -632,18 +632,35 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, goto unlock; wait_on_page_writeback(page); } - /* - * Establish migration ptes or remove ptes + * By try_to_unmap(), page->mapcount goes down to 0 here. In this case, + * we cannot notice that anon_vma is freed while we migrates a page. + * This rcu_read_lock() delays freeing anon_vma pointer until the end + * of migration. File cache pages are no problem because of page_lock() + */ + rcu_read_lock(); + /* + * This is a corner case handling. + * When a new swap-cache is read into, it is linked to LRU + * and treated as swapcache but has no rmap yet. + * Calling try_to_unmap() against a page->mapping==NULL page is + * BUG. So handle it here. */ + if (!page->mapping) + goto rcu_unlock; + /* Establish migration ptes or remove ptes */ try_to_unmap(page, 1); + if (!page_mapped(page)) rc = move_to_new_page(newpage, page); if (rc) remove_migration_ptes(page, page); +rcu_unlock: + rcu_read_unlock(); unlock: + unlock_page(page); if (rc != -EAGAIN) { -- cgit v1.2.3-59-g8ed1b