Commit dc386d4d authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds

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: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: default avatarChristoph Lameter <clameter@sgi.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 09828402
...@@ -632,18 +632,35 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, ...@@ -632,18 +632,35 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
goto unlock; goto unlock;
wait_on_page_writeback(page); 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); try_to_unmap(page, 1);
if (!page_mapped(page)) if (!page_mapped(page))
rc = move_to_new_page(newpage, page); rc = move_to_new_page(newpage, page);
if (rc) if (rc)
remove_migration_ptes(page, page); remove_migration_ptes(page, page);
rcu_unlock:
rcu_read_unlock();
unlock: unlock:
unlock_page(page); unlock_page(page);
if (rc != -EAGAIN) { if (rc != -EAGAIN) {
......
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