Commit dc386d4d authored Jul 26, 2007 by KAMEZAWA Hiroyuki Committed by Linus Torvalds Jul 26, 2007

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 <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 09828402

mm/migrate.c

+19 −2

Original line number	Diff line number	Diff line
		@@ -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) {