linux-kernel - Re: [PATCH v4 3/9] mm/lru: replace pgdat lru

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Date:   Wed, 20 Nov 2019 19:41:44 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, tj@...nel.org, hughd@...gle.com,
        khlebnikov@...dex-team.ru, daniel.m.jordan@...cle.com,
        yang.shi@...ux.alibaba.com, willy@...radead.org,
        shakeelb@...gle.com, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Chris Down <chris@...isdown.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, Qian Cai <cai@....pw>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        swkhack <swkhack@...il.com>,
        "Potyra, Stefan" <Stefan.Potyra@...ktrobit.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Colin Ian King <colin.king@...onical.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Peng Fan <peng.fan@....com>,
        Nikolay Borisov <nborisov@...e.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock



在 2019/11/20 上午12:04, Johannes Weiner 写道:
>> +
>> +	return lruvec;
> While this works in practice, it looks wrong because it doesn't follow
> the mem_cgroup_page_lruvec() rules.
> 
> Please open-code spin_lock_irq(&pgdat->__lruvec->lru_lock) instead.
> 

That's right. Thanks for suggestion!

>> @@ -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. 

> 
> Either the page needs to be locked, or the page needs to be PageLRU
> with the lru_lock held to prevent somebody else from isolating
> it. Otherwise, the lruvec is not safe to use.

Do you mean that we may get the wrong lruvec->lru_lock if !PageLRU, so, the page may got freed by others? Sorry I got last there.

Thanks
Alex