linux-kernel - Re: [PATCH block/for-next] writeback: add tracepoints for cgroup foreign writebacks

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Message-ID: <20190830154921.GZ2263813@devbig004.ftw2.facebook.com>
Date:   Fri, 30 Aug 2019 08:49:21 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH block/for-next] writeback: add tracepoints for cgroup
 foreign writebacks

Hello, Jan.

On Fri, Aug 30, 2019 at 05:40:23PM +0200, Jan Kara wrote:
> > +	TP_fast_assign(
> > +		strncpy(__entry->name,	dev_name(wb->bdi->dev), 32);
> > +		__entry->bdi_id		= wb->bdi->id;
> > +		__entry->ino		= page->mapping->host->i_ino;
> > +		__entry->memcg_id	= wb->memcg_css->id;
> > +		__entry->cgroup_ino	= __trace_wb_assign_cgroup(wb);
> > +		__entry->page_cgroup_ino = page->mem_cgroup->css.cgroup->kn->id.ino;
> > +	),
> 
> 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?

Hah, I assumed it would work because there are preceding if
(page_mapping()) tests in the dirty paths -
e.g. __set_page_dirty_nobuffers().  Oh, regardless of that assumption,
I should have used page_mapping().

Thanks.

-- 
tejun