linux-kernel - Re: [RFC PATCH 3/3] memcg-v1: remove memcg move locking code

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Message-ID: <ZxqXyQb1nT6DWPN_@google.com>
Date: Thu, 24 Oct 2024 18:54:01 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Michal Hocko <mhocko@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [RFC PATCH 3/3] memcg-v1: remove memcg move locking code

On Thu, Oct 24, 2024 at 10:23:49AM -0700, Shakeel Butt wrote:
> On Thu, Oct 24, 2024 at 11:16:52AM GMT, Michal Hocko wrote:
> > On Wed 23-10-24 23:57:12, Shakeel Butt wrote:
> > > The memcg v1's charge move feature has been deprecated. There is no need
> > > to have any locking or protection against the moving charge. Let's
> > > proceed to remove all the locking code related to charge moving.
> > > 
> > > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > > ---
> > > -/**
> > > - * folio_memcg_lock - Bind a folio to its memcg.
> > > - * @folio: The folio.
> > > - *
> > > - * This function prevents unlocked LRU folios from being moved to
> > > - * another cgroup.
> > > - *
> > > - * It ensures lifetime of the bound memcg.  The caller is responsible
> > > - * for the lifetime of the folio.
> > > - */
> > > -void folio_memcg_lock(struct folio *folio)
> > > -{
> > > -	struct mem_cgroup *memcg;
> > > -	unsigned long flags;
> > > -
> > > -	/*
> > > -	 * The RCU lock is held throughout the transaction.  The fast
> > > -	 * path can get away without acquiring the memcg->move_lock
> > > -	 * because page moving starts with an RCU grace period.
> > > -         */
> > > -	rcu_read_lock();
> > 
> > Is it safe to remove the implicit RCU?
> 
> Good question. I think it will be safe to keep the RCU in this patch and
> in the followup examine each place and decide to remove RCU or not.

I took a really quick look and based on it I believe it is safe.
Shakeel, can you, please, check too and preferably keep your code intact.
I think it's better to remove it all together, rather than do it in two steps.
If we really need rcu somewhere, we can replace folio_memcg_lock()/unlock()
with an explicit rcu_read_lock()/rcu_read_unlock().

Thanks!