linux-kernel - Re: [block/for-next] writeback: don't access page->mapping directly in track_foreign

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Message-ID: <20190902085628.GC14207@quack2.suse.cz>
Date:   Mon, 2 Sep 2019 10:56:28 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [block/for-next] writeback: don't access page->mapping directly
 in track_foreign_dirty TP

On Fri 30-08-19 16:39:54, Tejun Heo wrote:
> page->mapping may encode different values in it and page_mapping()
> should always be used to access the mapping pointer.
> track_foreign_dirty tracepoint was incorrectly accessing page->mapping
> directly.  Use page_mapping() instead.  Also, add NULL checks while at
> it.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Jan Kara <jack@...e.cz>
> Fixes: 3a8e9ac89e6a ("writeback: add tracepoints for cgroup foreign writebacks")

I can see Jens already picked this up so this is just informative: The patch
now looks good to me.

								Honza

> ---
>  include/trace/events/writeback.h |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index 3dc9fb9e7c78..3a27335fce2c 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -251,9 +251,12 @@ TRACE_EVENT(track_foreign_dirty,
>  	),
>  
>  	TP_fast_assign(
> +		struct address_space *mapping = page_mapping(page);
> +		struct inode *inode = mapping ? mapping->host : NULL;
> +
>  		strncpy(__entry->name,	dev_name(wb->bdi->dev), 32);
>  		__entry->bdi_id		= wb->bdi->id;
> -		__entry->ino		= page->mapping->host->i_ino;
> +		__entry->ino		= inode ? inode->i_ino : 0;
>  		__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;
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR