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Message-Id: <20251008000014.2845-1-sj@kernel.org>
Date: Tue, 7 Oct 2025 17:00:14 -0700
From: SeongJae Park <sj@...nel.org>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
damon@...ts.linux.dev,
kernel-team@...a.com,
linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [RFC PATCH 03/11] mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP
On Thu, 19 Jun 2025 15:00:15 -0700 SeongJae Park <sj@...nel.org> wrote:
> Implement the behavior of DAMOS_QUOTA_NODE_MEMCG_USED_BP. It uses sum
> of active/inactive anon/file pages of a given cgroup on a given NUMA
> node as the value of the metric.
>
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
> mm/damon/core.c | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index d675dd932a23..1481b43f2710 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
[...]
> +static unsigned long damos_get_node_memcg_used_bp(
> + struct damos_quota_goal *goal)
> +{
> + struct mem_cgroup *memcg;
> + struct lruvec *lruvec;
> + unsigned long used_pages;
> + struct sysinfo i;
> +
> + rcu_read_lock();
> + memcg = mem_cgroup_from_id(goal->memcg_id);
> + mem_cgroup_flush_stats(memcg);
> + lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(goal->nid));
> + used_pages = lruvec_page_state(lruvec, NR_ACTIVE_ANON);
> + used_pages += lruvec_page_state(lruvec, NR_INACTIVE_ANON);
> + used_pages += lruvec_page_state(lruvec, NR_ACTIVE_FILE);
> + used_pages += lruvec_page_state(lruvec, NR_INACTIVE_FILE);
> + rcu_read_unlock();
mem_cgroup_flush_stats() calls css_rstat_flush() which may block. But the
above diff is putting it inside an RCU read-side critical section. Actually
only mem_cgroup_from_id() needs to be called inside an RCU read-side critical
section. I will revise the next version to call rcu_read_unlock() right after
mem_cgroup_from_id().
Also, mem_cgroup_from_id() could return NULL if the memcg is removeed while
DAMON is running, but this diff is not checking the NULL return case. I will
add the handling in the next version.
> +
> + si_meminfo_node(&i, goal->nid);
> + return used_pages * PAGE_SIZE * 10000 / i.totalram;
i.totalram is in number of pages unit, but the above code is assuming it is in
bytes unit. This makes the result significantly wrong. I will fix this in the
next version.
Thanks,
SJ
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