Re: [PATCH v7 02/10] mm/memcg: fold lru_lock in lock_page_lru

On 13/01/2020 12.45, Alex Shi wrote:

在 2020/1/10 下午4:49, Konstantin Khlebnikov 写道:

quoted

On 25/12/2019 12.04, Alex Shi wrote:

quoted

  From the commit_charge's explanations and mem_cgroup_commit_charge
comments, as well as call path when lrucare is ture, The lru_lock is
just to guard the task migration(which would be lead to move_account)
So it isn't needed when !PageLRU, and better be fold into PageLRU to
reduce lock contentions.

Signed-off-by: Alex Shi <redacted>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Vladimir Davydov <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: cgroups@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
   mm/memcontrol.c | 9 ++++-----
   1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5b5f74cfd4d..0ad10caabc3d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2572,12 +2572,11 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
     static void lock_page_lru(struct page *page, int *isolated)
   {
-    pg_data_t *pgdat = page_pgdat(page);
-
-    spin_lock_irq(&pgdat->lru_lock);
       if (PageLRU(page)) {
+        pg_data_t *pgdat = page_pgdat(page);
           struct lruvec *lruvec;
   +        spin_lock_irq(&pgdat->lru_lock);

That's wrong. Here PageLRU must be checked again under lru_lock.

Hi, Konstantin,

For logical remain, we can get the lock and then release for !PageLRU.
but I still can figure out the problem scenario. Would like to give more hints?

That's trivial race: page could be isolated from lru between

if (PageLRU(page))
and
spin_lock_irq(&pgdat->lru_lock);

quoted

Also I don't like these functions:
- called lock/unlock but actually also isolates
- used just once
- pgdat evaluated twice

That's right. I will fold these functions into commit_charge.

Thanks
Alex