[PATCH 3/3] mm: strictly require elevated page refcount in isolate_lru_page()

Konstantin Khlebnikov
2011-04-21T13:12:54Z

isolate_lru_page() must be called only with stable reference to the page,
this is what is written in the comment above it, this is reasonable.

current isolate_lru_page() users and its page extra reference sources:

mm/huge_memory.c
__collapse_huge_page_isolate() - reference from pte

mm/memcontrol.c
mem_cgroup_move_parent() - get_page_unless_zero()
mem_cgroup_move_charge_pte_range() - reference from pte

mm/memory-failure.c
soft_offline_page() - fixed, reference from get_any_page()
delete_from_lru_cache() - reference from caller or get_page_unless_zero()
[seems like there bug, because __memory_failure() can call page_action() for
hpages tail, but it is ok for isolate_lru_page(), tail getted and not in lru]

mm/memory_hotplug.c
do_migrate_range() - fixed, get_page_unless_zero()

mm/mempolicy.c
migrate_page_add() - reference from pte

mm/migrate.c
do_move_page_to_node_array() - reference from follow_page()

mlock.c - various external references

mm/vmscan.c
putback_lru_page() - reference from isolate_lru_page()

It seems that all isolate_lru_page() users are ready now for this restriction.
So, let's replace redundant get_page_unless_zero() with get_page() and
add page initial reference count check with VM_BUG_ON()

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
---
mm/vmscan.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f6b435c..0175f39 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1201,13 +1201,16 @@ int isolate_lru_page(struct page *page)
{
int ret = -EBUSY;

+ VM_BUG_ON(!page_count(page));
+
if (PageLRU(page)) {
struct zone *zone = page_zone(page);

spin_lock_irq(&zone->lru_lock);
- if (PageLRU(page) && get_page_unless_zero(page)) {
+ if (PageLRU(page)) {
int lru = page_lru(page);
ret = 0;
+ get_page(page);
ClearPageLRU(page);

del_page_from_lru_list(zone, page, lru);