linux-kernel - Re: [PATCH mm-unstable v1 2/4] mm: increment gen # before restarting traversal

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Message-ID: <20240725205334.GB1702603@cmpxchg.org>
Date: Thu, 25 Jul 2024 16:53:34 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Kinsey Ho <kinseyho@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Yosry Ahmed <yosryahmed@...gle.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH mm-unstable v1 2/4] mm: increment gen # before restarting
 traversal

On Wed, Jul 24, 2024 at 07:02:12PM +0000, Kinsey Ho wrote:
> The generation number in struct mem_cgroup_reclaim_iter should be
> incremented on every round-trip. Currently, it is possible for a
> concurrent reclaimer to jump in at the end of the hierarchy, causing a
> traversal restart (resetting the iteration position) without
> incrementing the generation number.
> 
> Move the traversal restart such that the generation number is
> incremented before the restart.
> 
> Signed-off-by: Kinsey Ho <kinseyho@...gle.com>

The consequence of resetting the position without bumping the
generation would be that another ongoing mem_cgroup_iter() could walk
the tree twice, which is undesirable. It would be good to spell that
out in the changelog.

Otherwise, looks good to me.

Acked-by: Johannes Weiner <hannes@...xchg.org>