linux-kernel - Re: [PATCH] mm: memcg: fix swapcached stat accounting

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Message-ID: <Y43+cUNTrq3VKgU2@cmpxchg.org>
Date:   Mon, 5 Dec 2022 09:21:37 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Yu Zhao <yuzhao@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: memcg: fix swapcached stat accounting

On Sun, Dec 04, 2022 at 05:01:03PM -0800, Hugh Dickins wrote:
> I'd been worried by high "swapcached" counts in memcg OOM reports,
> thought we had a problem freeing swapcache, but it was just the
> accounting that was wrong.
> 
> Two issues:
> 
> 1. When __remove_mapping() removes swapcache, __delete_from_swap_cache()
> relies on memcg_data for the right counts to be updated; but that had
> already been reset by mem_cgroup_swapout().  Swap those calls around -
> mem_cgroup_swapout() does not require the swapcached flag to be set.
> 
> 6.1 commit ac35a4902374 ("mm: multi-gen LRU: minimal implementation")
> already made a similar swap for workingset_eviction(), but not for this.
> 
> 2. memcg's "swapcached" count was added for memcg v2 stats, but displayed
> on OOM even for memcg v1: so mem_cgroup_move_account() ought to move it.
> 
> Fixes: b6038942480e ("mm: memcg: add swapcache stat for memcg v2")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

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