On Sat, Mar 13, 2021 at 03:09:18PM +0000, Matthew Wilcox wrote: > 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.
I agree in principle. On this topic I often debate myself where to draw the line between being rigorous and paranoid. But in this particular case, I thought it's no brainer because, imo, most of the systems that don't use memcgs are small and preemptable, e.g., openwrt. They wouldn't appreciate a larger code size or rcu stalls due to preemptions of functions that take rcu locks just to be rigorous. This shouldn't be a problem if we only do so when CONFIG_DEBUG_VM=y, but then its test coverage is another question. I'd be happy to work out something in this direction, hopefully worth the trouble, if you think this compromise is acceptable.
