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Subject[PATCH 1/7] KAMEZAWA Hiroyuki hot-remove patches
Date

This is a rollup of two patches from KAMEZAWA Hiroyuki. A slightly later
version exists but this is the one I tested with and it checks page_mapped()
with the RCU lock held.

Patch 1 is "page migration by kernel v5."
Patch 2 is "isolate lru page race fix."

Changelog V5->V6
- removed dummy_vma and uses rcu_read_lock().

In usual, migrate_pages(page,,) is called with holoding mm->sem by systemcall.
(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 is not mapped can be target of migration. Then, we have
to check page_mapped() 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 will
not be freed during migration.

release_pages() in mm/swap.c changes page_count() to be 0
without removing PageLRU flag...

This means isolate_lru_page() can see a page, PageLRU() && page_count(page)==0..
This is BUG. (get_page() will be called against count=0 page.)

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
migrate.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)
diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.22-rc4-mm2-clean/mm/migrate.c linux-2.6.22-rc4-mm2-005_migrationkernel/mm/migrate.c
--- linux-2.6.22-rc4-mm2-clean/mm/migrate.c 2007-06-13 23:43:12.000000000 +0100
+++ linux-2.6.22-rc4-mm2-005_migrationkernel/mm/migrate.c 2007-06-15 16:25:31.000000000 +0100
@@ -49,9 +49,8 @@ int isolate_lru_page(struct page *page,
struct zone *zone = page_zone(page);

spin_lock_irq(&zone->lru_lock);
- if (PageLRU(page)) {
+ if (PageLRU(page) && get_page_unless_zero(page)) {
ret = 0;
- get_page(page);
ClearPageLRU(page);
if (PageActive(page))
del_page_from_active_list(zone, page);
@@ -612,6 +611,7 @@ static int unmap_and_move(new_page_t get
int rc = 0;
int *result = NULL;
struct page *newpage = get_new_page(page, private, &result);
+ int rcu_locked = 0;

if (!newpage)
return -ENOMEM;
@@ -632,18 +632,27 @@ static int unmap_and_move(new_page_t get
goto unlock;
wait_on_page_writeback(page);
}
-
+ /* anon_vma should not be freed while migration. */
+ if (PageAnon(page)) {
+ rcu_read_lock();
+ rcu_locked = 1;
+ }
/*
* Establish migration ptes or remove ptes
*/
- try_to_unmap(page, 1);
if (!page_mapped(page))
- rc = move_to_new_page(newpage, page);
+ goto unlock;
+
+ try_to_unmap(page, 1);
+ rc = move_to_new_page(newpage, page);

if (rc)
remove_migration_ptes(page, page);

unlock:
+ if (rcu_locked)
+ rcu_read_unlock();
+
unlock_page(page);

if (rc != -EAGAIN) {
-
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