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Wed, 25 May 2022 05:30:17 -0700 (PDT) Date: Wed, 25 May 2022 08:30:15 -0400 From: Johannes Weiner To: Muchun Song Cc: 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, longman@redhat.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-Rspam-User: X-Rspamd-Queue-Id: 5CD2F10000F X-Stat-Signature: dyguwr51kyjf7au6hymechwq1xahrdsj Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=cmpxchg-org.20210112.gappssmtp.com header.s=20210112 header.b=LQ44JAnh; spf=pass (imf14.hostedemail.com: domain of hannes@cmpxchg.org designates 209.85.160.174 as permitted sender) smtp.mailfrom=hannes@cmpxchg.org; dmarc=pass (policy=none) header.from=cmpxchg.org X-Rspamd-Server: rspam03 X-HE-Tag: 1653481815-902748 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 Wed, May 25, 2022 at 05:53:30PM +0800, Muchun Song wrote: > On Tue, May 24, 2022 at 03:27:20PM -0400, Johannes Weiner wrote: > > On Tue, May 24, 2022 at 02:05:43PM +0800, 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 > > > > This looks good to me. Just one question: > > > > > @@ -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. > > > + */ > > > + rcu_read_unlock(); > > > > The code looks right to me, but I don't understand the comment: why do > > we care that the rcu read-side continues? With the lru_lock held, > > reparenting is on hold and the lruvec cannot be rcu-freed anyway, no? > > > > Right. We could hold rcu read lock until end of reparting. So you mean > we do rcu_read_unlock in folio_lruvec_lock()? The comment seems to suggest that disabling preemption is what keeps the lruvec alive. But it's the lru_lock that keeps it alive. The cgroup destruction path tries to take the lru_lock long before it even gets to synchronize_rcu(). Once you hold the lru_lock, having an implied read-side critical section as well doesn't seem to matter. Should the comment be deleted?