Messages in this thread Patch in this message | | | Date | Fri, 6 Jul 2007 18:23:24 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | [PATCH] memory unplug v7 - migration by kernel |
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page migration by kernel v6.
Changelog V6->V7 - moved rcu_read_lock/rcu_read_unlock to correct place. - fixed text.
Changelog V5->V6 - removed dummy_vma and uses rcu_read_lock(). - removed page_mapped() check and uses !page->mapping check.
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>
--- mm/migrate.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-)
Index: linux-2.6.22-rc6-mm1/mm/migrate.c =================================================================== --- linux-2.6.22-rc6-mm1.orig/mm/migrate.c +++ linux-2.6.22-rc6-mm1/mm/migrate.c @@ -632,18 +632,35 @@ static int unmap_and_move(new_page_t get 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) { - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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