From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock Date: Thu, 21 Nov 2019 17:06:13 -0500 Message-ID: <20191121220613.GB487872@cmpxchg.org> References: <1574166203-151975-1-git-send-email-alex.shi@linux.alibaba.com> <1574166203-151975-4-git-send-email-alex.shi@linux.alibaba.com> <20191119160456.GD382712@cmpxchg.org> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to :user-agent; bh=Bf3zXVc8JgyndN70CrFb4zsWDVt1DI2cOBZZ54QutII=; b=crXpS16brbj5wcwSGg8ZUNW+WnFbky+6ndEsBKBBwqewLUyZC2KtPR06bmxf6kVV0x +XADaSG7XVEs8kl8hGMFWn6z4G/S2iNrNg+CMACl4gcVvkcOBydeqNL2Ns+YzhGMxGgS PoyNNpLZLqeT5y44r7ouxVEZyHJ+9xp1ZwgERCu3+/9v7foYL77J2/4FdM7VxPoiZyJa MQOkloOGTfuv5odbW2dAfcsnjD9UseKW3xuJTuBllUg9O4YX5BV24NY2VWje8XU4rsVe mSOlipeQetbcpiGdNL2OdQpEQoycht9RyfCrvg+Blyffk8wGm/bGCfNnrb2BBpuB75nf +Ufg== Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="utf-8" To: Alex Shi Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, mgorman@techsingularity.net, tj@kernel.org, hughd@google.com, khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com, yang.shi@linux.alibaba.com, willy@infradead.org, shakeelb@google.com, Michal Hocko , Vladimir Davydov , Roman Gushchin , Chris Down , Thomas Gleixner , Vlastimil Babka , Qian Cai , Andrey Ryabinin , "Kirill A. Shutemov" , =?iso-8859-1?B?Suly9G1l?= Glisse , Andrea Arcangeli , David Rientjes , Aneesh Ku On Wed, Nov 20, 2019 at 07:41:44PM +0800, Alex Shi wrote: > 在 2019/11/20 上午12:04, Johannes Weiner 写道: > >> @@ -1246,6 +1245,46 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd > >> return lruvec; > >> } > >> > >> +struct lruvec *lock_page_lruvec_irq(struct page *page, > >> + struct pglist_data *pgdat) > >> +{ > >> + struct lruvec *lruvec; > >> + > >> +again: > >> + rcu_read_lock(); > >> + lruvec = mem_cgroup_page_lruvec(page, pgdat); > >> + spin_lock_irq(&lruvec->lru_lock); > >> + rcu_read_unlock(); > > The spinlock doesn't prevent the lruvec from being freed > > > > You deleted the rules from the mem_cgroup_page_lruvec() documentation, > > but they still apply: if the page is already !PageLRU() by the time > > you get here, it could get reclaimed or migrated to another cgroup, > > and that can free the memcg/lruvec. Merely having the lru_lock held > > does not prevent this. > > > Forgive my idiot, I still don't know the details of unsafe lruvec here. > From my shortsight, the spin_lock_irq(embedded a preempt_disable) could block all rcu syncing thus, keep all memcg alive until the preempt_enabled in unspinlock, is this right? > If so even the page->mem_cgroup is migrated to others cgroups, the new and old cgroup should still be alive here. You are right about the freeing part, I missed this. And I should have read this email here before sending out my "fix" to the current code; thankfully Hugh re-iterated my mistake on that thread. My apologies. But I still don't understand how the moving part is safe. You look up the lruvec optimistically, lock it, then verify the lookup. What keeps page->mem_cgroup from changing after you verified it? lock_page_lruvec(): mem_cgroup_move_account(): again: rcu_read_lock() lruvec = page->mem_cgroup->lruvec isolate_lru_page() spin_lock_irq(&lruvec->lru_lock) rcu_read_unlock() if page->mem_cgroup->lruvec != lruvec: spin_unlock_irq(&lruvec->lru_lock) goto again; page->mem_cgroup = new cgroup putback_lru_page() // new lruvec SetPageLRU() return lruvec; // old lruvec The caller assumes page belongs to the returned lruvec and will then change the page's lru state with a mismatched page and lruvec. If we could restrict lock_page_lruvec() to working only on PageLRU pages, we could fix the problem with memory barriers. But this won't work for split_huge_page(), which is AFAICT the only user that needs to freeze the lru state of a page that could be isolated elsewhere. So AFAICS the only option is to lock out mem_cgroup_move_account() entirely when the lru_lock is held. Which I guess should be fine.