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Wed, 25 May 2022 03:20:30 -0700 (PDT) Date: Wed, 25 May 2022 18:20:25 +0800 From: Muchun Song To: Waiman Long Cc: hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeelb@google.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, duanxiongchun@bytedance.com Subject: Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented Message-ID: References: <20220524060551.80037-1-songmuchun@bytedance.com> <20220524060551.80037-4-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: D61304003C X-Stat-Signature: sspru74ynykr9xksh5o4ia6zpb31kb1w Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=bytedance-com.20210112.gappssmtp.com header.s=20210112 header.b=j79qceaO; spf=pass (imf04.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.215.181 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-Rspam-User: X-HE-Tag: 1653474016-847887 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, May 24, 2022 at 03:23:11PM -0400, Waiman Long wrote: > 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 > > --- > > 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? > I'm not familar with PREEMPT_RT kernel. At least you are right, preemption is not disabled in this case, I think I should drop this assumption. > > + 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. > Good point. I'll add it. Thanks. > > lru_note_cost(folio_lruvec(folio), folio_is_file_lru(folio), > > folio_nr_pages(folio)); > > } > > Cheers, > Longman > >