linux-kernel - Re: [PATCH mm-unstable v1 1/4] mm: don't hold css->refcnt during traversal

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Message-ID: <ZqK_CG9AtBy9mVVH@google.com>
Date: Thu, 25 Jul 2024 21:09:28 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Kinsey Ho <kinseyho@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Yosry Ahmed <yosryahmed@...gle.com>
Subject: Re: [PATCH mm-unstable v1 1/4] mm: don't hold css->refcnt during
 traversal

On Thu, Jul 25, 2024 at 04:43:46PM -0400, Johannes Weiner wrote:
> On Wed, Jul 24, 2024 at 07:02:11PM +0000, Kinsey Ho wrote:
> > To obtain the pointer to the saved memcg position, mem_cgroup_iter()
> > currently holds css->refcnt during memcg traversal only to put
> > css->refcnt at the end of the routine. This isn't necessary as an
> > rcu_read_lock is already held throughout the function.
> > 
> > Remove css->refcnt usage during traversal by leveraging RCU.
> 
> Eh, I don't know about this.
> 
> RCU ensures that the css memory isn't freed.
> 
> The tryget ensures that the css is still alive and valid.
> 
> In this case, it just so happens that the sibling linkage is also rcu
> protected. But accessing random css members when the refcount is 0 is
> kind of sketchy. On the other hand, the refcount is guaranteed to be
> valid, and rcu + tryget is a common pattern.

I also spent quite some time thinking about potential bad consequences,
but _it seems_ to be safe (but I agree it feels dangerous).

> 
> What does this buy us? The tryget is cheap.

To be fair, tryget is not always cheap. Offline/dying cgroups have an atomic
operation there.