Re: [Patch v3] mm: thp: grab the lock before manipulation defer list

On 16.01.2020 04:31, Wei Yang wrote:

As all the other places, we grab the lock before manipulate the defer list. Current implementation may face a race condition.

For example, the potential race would be:

CPU1                      CPU2
mem_cgroup_move_account   deferred_split_huge_page
  list_empty
                            lock
                            list_empty
                            list_add_tail
                            unlock
  lock
  # list_empty might not hold anymore
  list_add_tail
  unlock

When this sequence happens, the list_add_tail() in mem_cgroup_move_account() corrupt the list since which is already been added to some split_queue in split_huge_page_to_list().

Besides this, David Rientjes points out the split_queue_len would be in a wrong state, which would be a significant issue for shrinkers.

Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")

Signed-off-by: Wei Yang richardw.yang@linux.intel.com Cc: stable@vger.kernel.org [5.4+]

---

v3:

• remove all review/ack tag since rewrite the changelog
• use deferred_split_huge_page as the example of race
• add cc stable 5.4+ tag as suggested by David Rientjes

v2:

• move check on compound outside suggested by Alexander
• an example of the race condition, suggested by Michal

---

mm/memcontrol.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c index c5b5f74cfd4d..6450bbe394e2 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5360,10 +5360,12 @@ static int mem_cgroup_move_account(struct page *page, } #ifdef CONFIG_TRANSPARENT_HUGEPAGE

• if (compound && !list_empty(page_deferred_list(page))) {

• if (compound) { spin_lock(&from->deferred_split_queue.split_queue_lock);

• list_del_init(page_deferred_list(page));
  

• from->deferred_split_queue.split_queue_len--;
  

• if (!list_empty(page_deferred_list(page))) {
  

• 	list_del_init(page_deferred_list(page));
  

• 	from->deferred_split_queue.split_queue_len--;
  

• }
  

  spin_unlock(&from->deferred_split_queue.split_queue_lock); }

#endif @@ -5377,11 +5379,13 @@ static int mem_cgroup_move_account(struct page *page, page->mem_cgroup = to; #ifdef CONFIG_TRANSPARENT_HUGEPAGE

• if (compound && list_empty(page_deferred_list(page))) {

• if (compound) { spin_lock(&to->deferred_split_queue.split_queue_lock);

• list_add_tail(page_deferred_list(page),
  

• 	      &to->deferred_split_queue.split_queue);
  

• to->deferred_split_queue.split_queue_len++;
  

• if (list_empty(page_deferred_list(page))) {
  

• 	list_add_tail(page_deferred_list(page),
  

• 		      &to->deferred_split_queue.split_queue);
  

• 	to->deferred_split_queue.split_queue_len++;
  

• }
  

  spin_unlock(&to->deferred_split_queue.split_queue_lock); }

#endif

The patch looks OK for me. But there is another question. I forget, why we unconditionally add a page with empty deferred list to deferred_split_queue. Shouldn't we also check that it was initially in the list? Something like:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c index d4394ae4e5be..0be0136adaa6 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5289,6 +5289,7 @@ static int mem_cgroup_move_account(struct page *page, struct pglist_data *pgdat; unsigned long flags; unsigned int nr_pages = compound ? hpage_nr_pages(page) : 1; + bool split = false; int ret; bool anon;

@@ -5346,6 +5347,7 @@ static int mem_cgroup_move_account(struct page *page, if (!list_empty(page_deferred_list(page))) { list_del_init(page_deferred_list(page)); from->deferred_split_queue.split_queue_len--; + split = true; } spin_unlock(&from->deferred_split_queue.split_queue_lock); } @@ -5360,7 +5362,7 @@ static int mem_cgroup_move_account(struct page *page, page->mem_cgroup = to;

#ifdef CONFIG_TRANSPARENT_HUGEPAGE - if (compound) { + if (compound && split) { spin_lock(&to->deferred_split_queue.split_queue_lock); if (list_empty(page_deferred_list(page))) { list_add_tail(page_deferred_list(page),