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Date:   Fri, 30 Aug 2019 08:51:01 -0700
From:   Tejun Heo <tj@kernel.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH block/for-next] writeback: add tracepoints for cgroup
 foreign writebacks
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 <20190830154023.GC25069@quack2.suse.cz>
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On Fri, Aug 30, 2019 at 05:40:23PM +0200, Jan Kara wrote:
> Are the page dereferences above safe? I suppose lock_page_memcg() protects
> the page->mem_cgroup->css.cgroup->kn->id dereference? But page->mapping
> does not seem to be protected by page lock?

Forgot to respond to the mem_cgroup part.  The page to memcg
association never changes on cgroup2 as long as the page has ref, so I
think that one should be safe.

Thanks.

-- 
tejun