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Date: Tue, 22 Nov 2022 15:31:24 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Shakeel Butt <shakeelb@...gle.com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...e.com>, Yu Zhao <yuzhao@...gle.com>,
Muchun Song <songmuchun@...edance.com>,
David Rientjes <rientjes@...gle.com>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
Vasily Averin <vasily.averin@...ux.dev>,
Vlastimil Babka <vbabka@...e.cz>,
Chris Down <chris@...isdown.name>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: memcg: fix stale protection of reclaim target memcg
+David Rientjes
The attached test reproduces the problem on a cgroup v2 hierarchy
mounted with memory_recursiveprot, and fails without this patch.
On Tue, Nov 22, 2022 at 3:27 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> During reclaim, mem_cgroup_calculate_protection() is used to determine
> the effective protection (emin and elow) values of a memcg. The
> protection of the reclaim target is ignored, but we cannot set their
> effective protection to 0 due to a limitation of the current
> implementation (see comment in mem_cgroup_protection()). Instead,
> we leave their effective protection values unchaged, and later ignore it
> in mem_cgroup_protection().
>
> However, mem_cgroup_protection() is called later in
> shrink_lruvec()->get_scan_count(), which is after the
> mem_cgroup_below_{min/low}() checks in shrink_node_memcgs(). As a
> result, the stale effective protection values of the target memcg may
> lead us to skip reclaiming from the target memcg entirely, before
> calling shrink_lruvec(). This can be even worse with recursive
> protection, where the stale target memcg protection can be higher than
> its standalone protection.
>
> An example where this can happen is as follows. Consider the following
> hierarchy with memory_recursiveprot:
> ROOT
> |
> A (memory.min = 50M)
> |
> B (memory.min = 10M, memory.high = 40M)
>
> Consider the following scenarion:
> - B has memory.current = 35M.
> - The system undergoes global reclaim (target memcg is NULL).
> - B will have an effective min of 50M (all of A's unclaimed protection).
> - B will not be reclaimed from.
> - Now allocate 10M more memory in B, pushing it above it's high limit.
> - The system undergoes memcg reclaim from B (target memcg is B)
> - In shrink_node_memcgs(), we call mem_cgroup_calculate_protection(),
> which immediately returns for B without doing anything, as B is the
> target memcg, relying on mem_cgroup_protection() to ignore B's stale
> effective min (still 50M).
> - Directly after mem_cgroup_calculate_protection(), we will call
> mem_cgroup_below_min(), which will read the stale effective min for B
> and skip it (instead of ignoring its protection as intended). In this
> case, it's really bad because we are not just considering B's
> standalone protection (10M), but we are reading a much higher stale
> protection (50M) which will cause us to not reclaim from B at all.
>
> This is an artifact of commit 45c7f7e1ef17 ("mm, memcg: decouple
> e{low,min} state mutations from protection checks") which made
> mem_cgroup_calculate_protection() only change the state without
> returning any value. Before that commit, we used to return
> MEMCG_PROT_NONE for the target memcg, which would cause us to skip the
> mem_cgroup_below_{min/low}() checks. After that commit we do not return
> anything and we end up checking the min & low effective protections for
> the target memcg, which are stale.
>
> Add mem_cgroup_ignore_protection() that checks if we are reclaiming from
> the target memcg, and call it in mem_cgroup_below_{min/low}() to ignore
> the stale protection of the target memcg.
>
> Fixes: 45c7f7e1ef17 ("mm, memcg: decouple e{low,min} state mutations from protection checks")
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
> include/linux/memcontrol.h | 33 +++++++++++++++++++++++++++------
> mm/vmscan.c | 11 ++++++-----
> 2 files changed, 33 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e1644a24009c..22c9c9f9c6b1 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -625,18 +625,32 @@ static inline bool mem_cgroup_supports_protection(struct mem_cgroup *memcg)
>
> }
>
> -static inline bool mem_cgroup_below_low(struct mem_cgroup *memcg)
> +static inline bool mem_cgroup_ignore_protection(struct mem_cgroup *target,
> + struct mem_cgroup *memcg)
> {
> - if (!mem_cgroup_supports_protection(memcg))
> + /*
> + * The target memcg's protection is ignored, see
> + * mem_cgroup_calculate_protection() and mem_cgroup_protection()
> + */
> + return target == memcg;
> +}
> +
> +static inline bool mem_cgroup_below_low(struct mem_cgroup *target,
> + struct mem_cgroup *memcg)
> +{
> + if (!mem_cgroup_supports_protection(memcg) ||
> + mem_cgroup_ignore_protection(target, memcg))
> return false;
>
> return READ_ONCE(memcg->memory.elow) >=
> page_counter_read(&memcg->memory);
> }
>
> -static inline bool mem_cgroup_below_min(struct mem_cgroup *memcg)
> +static inline bool mem_cgroup_below_min(struct mem_cgroup *target,
> + struct mem_cgroup *memcg)
> {
> - if (!mem_cgroup_supports_protection(memcg))
> + if (!mem_cgroup_supports_protection(memcg) ||
> + mem_cgroup_ignore_protection(target, memcg))
> return false;
>
> return READ_ONCE(memcg->memory.emin) >=
> @@ -1209,12 +1223,19 @@ static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
> {
> }
>
> -static inline bool mem_cgroup_below_low(struct mem_cgroup *memcg)
> +static inline bool mem_cgroup_ignore_protection(struct mem_cgroup *target,
> + struct mem_cgroup *memcg)
> +{
> + return false;
> +}
> +static inline bool mem_cgroup_below_low(struct mem_cgroup *target,
> + struct mem_cgroup *memcg)
> {
> return false;
> }
>
> -static inline bool mem_cgroup_below_min(struct mem_cgroup *memcg)
> +static inline bool mem_cgroup_below_min(struct mem_cgroup *target,
> + struct mem_cgroup *memcg)
> {
> return false;
> }
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 04d8b88e5216..79ef0fe67518 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4486,7 +4486,7 @@ static bool age_lruvec(struct lruvec *lruvec, struct scan_control *sc, unsigned
>
> mem_cgroup_calculate_protection(NULL, memcg);
>
> - if (mem_cgroup_below_min(memcg))
> + if (mem_cgroup_below_min(NULL, memcg))
> return false;
>
> need_aging = should_run_aging(lruvec, max_seq, min_seq, sc, swappiness, &nr_to_scan);
> @@ -5047,8 +5047,9 @@ static unsigned long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *
> DEFINE_MAX_SEQ(lruvec);
> DEFINE_MIN_SEQ(lruvec);
>
> - if (mem_cgroup_below_min(memcg) ||
> - (mem_cgroup_below_low(memcg) && !sc->memcg_low_reclaim))
> + if (mem_cgroup_below_min(sc->target_mem_cgroup, memcg) ||
> + (mem_cgroup_below_low(sc->target_mem_cgroup, memcg) &&
> + !sc->memcg_low_reclaim))
> return 0;
>
> *need_aging = should_run_aging(lruvec, max_seq, min_seq, sc, can_swap, &nr_to_scan);
> @@ -6048,13 +6049,13 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>
> mem_cgroup_calculate_protection(target_memcg, memcg);
>
> - if (mem_cgroup_below_min(memcg)) {
> + if (mem_cgroup_below_min(target_memcg, memcg)) {
> /*
> * Hard protection.
> * If there is no reclaimable memory, OOM.
> */
> continue;
> - } else if (mem_cgroup_below_low(memcg)) {
> + } else if (mem_cgroup_below_low(target_memcg, memcg)) {
> /*
> * Soft protection.
> * Respect the protection only as long as
> --
> 2.38.1.584.g0f3c55d4c2-goog
>
View attachment "test.sh" of type "text/x-sh" (3672 bytes)
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