Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=dc386d4d1e98bb39fb967ee156cd456c802fc692
Commit: dc386d4d1e98bb39fb967ee156cd456c802fc692
Parent: 098284020c47c1212d211e39ae2b41c21182e056
Author: KAMEZAWA Hiroyuki <[EMAIL PROTECTED]>
AuthorDate: Thu Jul 26 10:41:07 2007 -0700
Committer: Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Thu Jul 26 11:35:17 2007 -0700
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 <[EMAIL PROTECTED]>
Acked-by: Christoph Lameter <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
---
mm/migrate.c | 21 +++++++++++++++++++--
1 files changed, 19 insertions(+), 2 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 34d8ada..c8d8722 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) {
-
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