Re: [PATCH v21 17/19] mm/lru: replace pgdat lru_lock with lruvec lock

[Vlastimil Babka <vbabka@suse.cz>]

On 11/5/20 9:55 AM, Alex Shi wrote:
> This patch moves per node lru_lock into lruvec, thus bring a lru_lock for
> each of memcg per node. So on a large machine, each of memcg don't
> have to suffer from per node pgdat->lru_lock competition. They could go
> fast with their self lru_lock.
> 
> After move memcg charge before lru inserting, page isolation could
> serialize page's memcg, then per memcg lruvec lock is stable and could
> replace per node lru lock.
> 
> In func isolate_migratepages_block, compact_unlock_should_abort and
> lock_page_lruvec_irqsave are open coded to work with compact_control.
> Also add a debug func in locking which may give some clues if there are
> sth out of hands.
> 
> Daniel Jordan's testing show 62% improvement on modified readtwice case
> on his 2P * 10 core * 2 HT broadwell box.
> https://lore.kernel.org/lkml/20200915165807.kpp7uhiw7l3loofu@ca-dmjordan1.us.oracle.com/
> 
> On a large machine with memcg enabled but not used, the page's lruvec
> seeking pass a few pointers, that may lead to lru_lock holding time
> increase and a bit regression.
> 
> Hugh Dickins helped on the patch polish, thanks!
> 
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Acked-by: Hugh Dickins <hughd@google.com>
> Cc: Rong Chen <rong.a.chen@intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: cgroups@vger.kernel.org

I think I need some explanation about the rcu_read_lock() usage in 
lock_page_lruvec*() (and places effectively opencoding it).
Preferably in form of some code comment, but that can be also added as a 
additional patch later, I don't want to block the series.

mem_cgroup_page_lruvec() comment says

  * This function relies on page->mem_cgroup being stable - see the
  * access rules in commit_charge().

commit_charge() comment:

          * Any of the following ensures page->mem_cgroup stability:
          *
          * - the page lock
          * - LRU isolation
          * - lock_page_memcg()
          * - exclusive reference

"LRU isolation" used to be quite clear, but now is it after 
TestClearPageLRU(page) or after deleting from the lru list as well?
Also it doesn't mention rcu_read_lock(), should it?

So what exactly are we protecting by rcu_read_lock() in e.g. lock_page_lruvec()?

         rcu_read_lock();
         lruvec = mem_cgroup_page_lruvec(page, pgdat);
         spin_lock(&lruvec->lru_lock);
         rcu_read_unlock();

Looks like we are protecting the lruvec from going away and it can't go away 
anymore after we take the lru_lock?

But then e.g. in __munlock_pagevec() we are doing this without an rcu_read_lock():

	new_lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));

where new_lruvec is potentionally not the one that we have locked

And the last thing mem_cgroup_page_lruvec() is doing is:

         if (unlikely(lruvec->pgdat != pgdat))
                 lruvec->pgdat = pgdat;
         return lruvec;

So without the rcu_read_lock() is this potentionally accessing the pgdat field 
of lruvec that might have just gone away?

Thanks,
Vlastimil