Re: [PATCH v2] mm/list_lru: Fix possible race in memcg_reparent_list_lru_node()

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 31 Mar 2022 09:46:52 +0200 Michal Hocko <mhocko@suse.com> wrote:

> On Thu 31-03-22 06:39:56, Shakeel Butt wrote:
> > On Wed, Mar 30, 2022 at 07:48:45PM -0700, Roman Gushchin wrote:
> > > 
> > > 
> > [...]
> > > 
> > > 
> > > But honestly, I’d drop the original optimization together with
> > > the fix, if only there is no _real world_ data on the problem and
> > > the improvement. It seems like it has started as a nice simple
> > > improvement, but the race makes it complex and probably not worth
> > > the added complexity and fragility.
> > 
> > I agree with dropping the original optimization as it is not really
> > fixing an observed issue which may justify adding some complexity.
> 
> Completely agreed. The patch as it is proposed is not really acceptable
> IMHO and I have to say I am worried that this is not the first time we
> are in a situation when a follow up fixes or unrelated patches are
> growing in complexity to fit on top of a performance optimizations which
> do not refer to any actual numbers.

Yup.  I did this:

From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm/list_lru.c: revert "mm/list_lru: optimize memcg_reparent_list_lru_node()"

405cc51fc1049c73 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
has subtle races which are proving ugly to fix.  Revert the original
optimization.  If quantitative testing indicates that we have a
significant problem here then other implementations can be looked at.

Fixes: 405cc51fc1049c73 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
Cc: Waiman Long <longman@redhat.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/list_lru.c |    6 ------
 1 file changed, 6 deletions(-)

--- a/mm/list_lru.c~revert-1
+++ a/mm/list_lru.c
@@ -395,12 +395,6 @@ static void memcg_reparent_list_lru_node
 	struct list_lru_one *src, *dst;
 
 	/*
-	 * If there is no lru entry in this nlru, we can skip it immediately.
-	 */
-	if (!READ_ONCE(nlru->nr_items))
-		return;
-
-	/*
 	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
 	 * we have to use IRQ-safe primitives here to avoid deadlock.
 	 */
_