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Wed, 25 May 2022 10:59:59 -0400 X-MC-Unique: 8h6aK_FZN4-QwsgJpwB8PQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id C69DB85A5BC; Wed, 25 May 2022 14:59:58 +0000 (UTC) Received: from [10.22.34.172] (unknown [10.22.34.172]) by smtp.corp.redhat.com (Postfix) with ESMTP id 469ED8287E; Wed, 25 May 2022 14:59:58 +0000 (UTC) Message-ID: <0728780f-08e7-3d09-e859-aeec92a81550@redhat.com> Date: Wed, 25 May 2022 10:59:58 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.8.0 Subject: Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented Content-Language: en-US To: Muchun Song 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 References: <20220524060551.80037-1-songmuchun@bytedance.com> <20220524060551.80037-4-songmuchun@bytedance.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Rspam-User: X-Rspamd-Queue-Id: 991514003B X-Stat-Signature: 4sjzw8bezqu4idhbm7pf969jbmk8k4a9 Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=bg31M50F; spf=none (imf07.hostedemail.com: domain of longman@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=longman@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam03 X-HE-Tag: 1653490792-212440 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 5/25/22 06:20, Muchun Song wrote: > 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. Preemption is not disabled for PREEMPT_RT kernel but task migration to another cpu is disabled. So access to per-cpu variables are safe. RCU seems to have a special mode for PREEMPT_RT kernel but I am not familiar with the detail. Cheers, Longman