On Sat, Mar 13, 2021 at 12:57:34AM -0700, Yu Zhao wrote: > We want to make sure the rcu lock is held while using > page_memcg_rcu(). But having a WARN_ON_ONCE() in page_memcg_rcu() when > !CONFIG_MEMCG is superfluous because of the following legit use case: > > memcg = lock_page_memcg(page1) > (rcu_read_lock() if CONFIG_MEMCG=y) > > do something to page1 > > if (page_memcg_rcu(page2) == memcg) > do something to page2 too as it cannot be migrated away from the > memcg either. > > unlock_page_memcg(page1) > (rcu_read_unlock() if CONFIG_MEMCG=y) > > This patch removes the WARN_ON_ONCE() from page_memcg_rcu() for the > !CONFIG_MEMCG case. I think this is wrong. Usually we try to have the same locking environment no matter what the CONFIG options are, like with kmap_atomic(). I think lock_page_memcg() should disable RCU even if CONFIG_MEMCG=n.
