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

[Jan Kara]

On Thu 29-08-19 15:47:19, Tejun Heo wrote:
> cgroup foreign inode handling has quite a bit of heuristics and
> internal states which sometimes makes it difficult to understand
> what's going on.  Add tracepoints to improve visibility.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
...
> +TRACE_EVENT(track_foreign_dirty,
> +
> +	TP_PROTO(struct page *page, struct bdi_writeback *wb),
> +
> +	TP_ARGS(page, wb),
> +
> +	TP_STRUCT__entry(
> +		__array(char,		name, 32)
> +		__field(u64,		bdi_id)
> +		__field(unsigned long,	ino)
> +		__field(unsigned int,	memcg_id)
> +		__field(unsigned int,	cgroup_ino)
> +		__field(unsigned int,	page_cgroup_ino)
> +	),
> +
> +	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?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR