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Date: Tue, 24 May 2022 15:23:11 -0400
From: Waiman Long <longman@...hat.com>
To: Muchun Song <songmuchun@...edance.com>, hannes@...xchg.org,
mhocko@...nel.org, roman.gushchin@...ux.dev, shakeelb@...gle.com
Cc: cgroups@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, duanxiongchun@...edance.com
Subject: Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU
pages are reparented
On 5/24/22 02:05, Muchun Song wrote:
> The diagram below shows how to make the folio lruvec lock safe when LRU
> pages are reparented.
>
> folio_lruvec_lock(folio)
> retry:
> lruvec = folio_lruvec(folio);
>
> // The folio is reparented at this time.
> spin_lock(&lruvec->lru_lock);
>
> if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio)))
> // Acquired the wrong lruvec lock and need to retry.
> // Because this folio is on the parent memcg lruvec list.
> goto retry;
>
> // If we reach here, it means that folio_memcg(folio) is stable.
>
> memcg_reparent_objcgs(memcg)
> // lruvec belongs to memcg and lruvec_parent belongs to parent memcg.
> spin_lock(&lruvec->lru_lock);
> spin_lock(&lruvec_parent->lru_lock);
>
> // Move all the pages from the lruvec list to the parent lruvec list.
>
> spin_unlock(&lruvec_parent->lru_lock);
> spin_unlock(&lruvec->lru_lock);
>
> After we acquire the lruvec lock, we need to check whether the folio is
> reparented. If so, we need to reacquire the new lruvec lock. On the
> routine of the LRU pages reparenting, we will also acquire the lruvec
> lock (will be implemented in the later patch). So folio_memcg() cannot
> be changed when we hold the lruvec lock.
>
> Since lruvec_memcg(lruvec) is always equal to folio_memcg(folio) after
> we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So
> remove it.
>
> This is a preparation for reparenting the LRU pages.
>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
> include/linux/memcontrol.h | 18 +++-----------
> mm/compaction.c | 10 +++++++-
> mm/memcontrol.c | 62 +++++++++++++++++++++++++++++-----------------
> mm/swap.c | 4 +++
> 4 files changed, 55 insertions(+), 39 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index ff1c1dd7e762..4042e4d21fe2 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -752,7 +752,9 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
> * folio_lruvec - return lruvec for isolating/putting an LRU folio
> * @folio: Pointer to the folio.
> *
> - * This function relies on folio->mem_cgroup being stable.
> + * The lruvec can be changed to its parent lruvec when the page reparented.
> + * The caller need to recheck if it cares about this changes (just like
> + * folio_lruvec_lock() does).
> */
> static inline struct lruvec *folio_lruvec(struct folio *folio)
> {
> @@ -771,15 +773,6 @@ struct lruvec *folio_lruvec_lock_irq(struct folio *folio);
> struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
> unsigned long *flags);
>
> -#ifdef CONFIG_DEBUG_VM
> -void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio);
> -#else
> -static inline
> -void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
> -{
> -}
> -#endif
> -
> static inline
> struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *css){
> return css ? container_of(css, struct mem_cgroup, css) : NULL;
> @@ -1240,11 +1233,6 @@ static inline struct lruvec *folio_lruvec(struct folio *folio)
> return &pgdat->__lruvec;
> }
>
> -static inline
> -void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
> -{
> -}
> -
> static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
> {
> return NULL;
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 817098817302..1692b17db781 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -515,6 +515,8 @@ compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags,
> {
> struct lruvec *lruvec;
>
> + rcu_read_lock();
> +retry:
> lruvec = folio_lruvec(folio);
>
> /* Track if the lock is contended in async mode */
> @@ -527,7 +529,13 @@ compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags,
>
> spin_lock_irqsave(&lruvec->lru_lock, *flags);
> out:
> - lruvec_memcg_debug(lruvec, folio);
> + if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
> + spin_unlock_irqrestore(&lruvec->lru_lock, *flags);
> + goto retry;
> + }
> +
> + /* See the comments in folio_lruvec_lock(). */
> + rcu_read_unlock();
>
> return lruvec;
> }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6de0d3e53eb1..b38a77f6696f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1199,23 +1199,6 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
> return ret;
> }
>
> -#ifdef CONFIG_DEBUG_VM
> -void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
> -{
> - struct mem_cgroup *memcg;
> -
> - if (mem_cgroup_disabled())
> - return;
> -
> - memcg = folio_memcg(folio);
> -
> - if (!memcg)
> - VM_BUG_ON_FOLIO(lruvec_memcg(lruvec) != root_mem_cgroup, folio);
> - else
> - VM_BUG_ON_FOLIO(lruvec_memcg(lruvec) != memcg, folio);
> -}
> -#endif
> -
> /**
> * folio_lruvec_lock - Lock the lruvec for a folio.
> * @folio: Pointer to the folio.
> @@ -1230,10 +1213,23 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
> */
> struct lruvec *folio_lruvec_lock(struct folio *folio)
> {
> - struct lruvec *lruvec = folio_lruvec(folio);
> + struct lruvec *lruvec;
>
> + rcu_read_lock();
> +retry:
> + lruvec = folio_lruvec(folio);
> spin_lock(&lruvec->lru_lock);
> - lruvec_memcg_debug(lruvec, folio);
> +
> + if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
> + spin_unlock(&lruvec->lru_lock);
> + goto retry;
> + }
> +
> + /*
> + * Preemption is disabled in the internal of spin_lock, which can serve
> + * as RCU read-side critical sections.
> + */
What is the point of this comment as preemption is not disabled for
PREEMPT_RT kernel?
> + rcu_read_unlock();
>
> return lruvec;
> }
> @@ -1253,10 +1249,20 @@ struct lruvec *folio_lruvec_lock(struct folio *folio)
> */
> struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
> {
> - struct lruvec *lruvec = folio_lruvec(folio);
> + struct lruvec *lruvec;
>
> + rcu_read_lock();
> +retry:
> + lruvec = folio_lruvec(folio);
> spin_lock_irq(&lruvec->lru_lock);
> - lruvec_memcg_debug(lruvec, folio);
> +
> + if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
> + spin_unlock_irq(&lruvec->lru_lock);
> + goto retry;
> + }
> +
> + /* See the comments in folio_lruvec_lock(). */
> + rcu_read_unlock();
>
> return lruvec;
> }
> @@ -1278,10 +1284,20 @@ struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
> struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
> unsigned long *flags)
> {
> - struct lruvec *lruvec = folio_lruvec(folio);
> + struct lruvec *lruvec;
>
> + rcu_read_lock();
> +retry:
> + lruvec = folio_lruvec(folio);
> spin_lock_irqsave(&lruvec->lru_lock, *flags);
> - lruvec_memcg_debug(lruvec, folio);
> +
> + if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
> + spin_unlock_irqrestore(&lruvec->lru_lock, *flags);
> + goto retry;
> + }
> +
> + /* See the comments in folio_lruvec_lock(). */
> + rcu_read_unlock();
>
> return lruvec;
> }
> diff --git a/mm/swap.c b/mm/swap.c
> index 7e320ec08c6a..9680f2fc48b1 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -303,6 +303,10 @@ void lru_note_cost(struct lruvec *lruvec, bool file, unsigned int nr_pages)
>
> void lru_note_cost_folio(struct folio *folio)
> {
> + /*
> + * The rcu read lock is held by the caller, so we do not need to
> + * care about the lruvec returned by folio_lruvec() being released.
> + */
Maybe we can add "WARN_ON_ONCE(!rcu_read_lock_held())" to be sure.
> lru_note_cost(folio_lruvec(folio), folio_is_file_lru(folio),
> folio_nr_pages(folio));
> }
Cheers,
Longman
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