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SubjectRe: [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@suse.com>
SUSE Labs, CR

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