Re: [PATCH v1 01/14] include/linux/memcontrol.h: do not warn in page_memcg_rcu() if !CONFIG_MEMCG

[Matthew Wilcox]

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.