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

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Date:   Fri, 30 Aug 2019 18:42:11 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, 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

On Fri 30-08-19 08:49:21, Tejun Heo wrote:
> 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().

Well, but if you look at __set_page_dirty_nobuffers() it is careful. It
does:

struct address_space *mapping = page_mapping(page);

if (!mapping) {
	bail
}
... use mapping

Exactly because page->mapping can become NULL under your hands if you don't
hold page lock. So I think you either need something similar in your
tracepoint or handle this in the caller.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR