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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>
This commit is contained in:
KAMEZAWA Hiroyuki 2007-07-26 10:41:07 -07:00 committed by Linus Torvalds
parent 098284020c
commit dc386d4d1e
1 changed files with 19 additions and 2 deletions
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21
mm/migrate.c
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@ -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) {