mm/memcg: remove rcu locking for lock_page_lruvec function series

lock_page_lruvec() and its variants used rcu_read_lock() with the
intention of safeguarding against the mem_cgroup being destroyed
concurrently; but so long as they are called under the specified
conditions (as they are), there is no way for the page's mem_cgroup to be
destroyed.  Delete the unnecessary rcu_read_lock() and _unlock().

Hugh Dickins polished the commit log.  Thanks a lot!

Link: https://lkml.kernel.org/r/1608614453-10739-2-git-send-email-alex.shi@linux.alibaba.com
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2b13b2cf5dcb86552bc891708c3c72b66d81dca3..5435b370c9294e8e8b8af9f6c1cc153d64adc8f4 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1357,10 +1357,8 @@ struct lruvec *lock_page_lruvec(struct page *page)
        struct lruvec *lruvec;
        struct pglist_data *pgdat = page_pgdat(page);
 
-       rcu_read_lock();
        lruvec = mem_cgroup_page_lruvec(page, pgdat);
        spin_lock(&lruvec->lru_lock);
-       rcu_read_unlock();
 
        lruvec_memcg_debug(lruvec, page);
 
@@ -1372,10 +1370,8 @@ struct lruvec *lock_page_lruvec_irq(struct page *page)
        struct lruvec *lruvec;
        struct pglist_data *pgdat = page_pgdat(page);
 
-       rcu_read_lock();
        lruvec = mem_cgroup_page_lruvec(page, pgdat);
        spin_lock_irq(&lruvec->lru_lock);
-       rcu_read_unlock();
 
        lruvec_memcg_debug(lruvec, page);
 
@@ -1387,10 +1383,8 @@ struct lruvec *lock_page_lruvec_irqsave(struct page *page, unsigned long *flags)
        struct lruvec *lruvec;
        struct pglist_data *pgdat = page_pgdat(page);
 
-       rcu_read_lock();
        lruvec = mem_cgroup_page_lruvec(page, pgdat);
        spin_lock_irqsave(&lruvec->lru_lock, *flags);
-       rcu_read_unlock();
 
        lruvec_memcg_debug(lruvec, page);