Re: kernel panic on null pointer on page->mem_cgroup

[Johannes Weiner]

On Thu, Aug 10, 2017 at 01:56:05PM +0200, Michal Hocko wrote:
> On Wed 09-08-17 14:38:25, Johannes Weiner wrote:
> > The issue is that writeback doesn't hold a page reference and the page
> > might get freed after PG_writeback is cleared (and the mapping is
> > unlocked) in test_clear_page_writeback(). The stat functions looking
> > up the page's node or zone are safe, as those attributes are static
> > across allocation and free cycles. But page->mem_cgroup is not, and it
> > will get cleared if we race with truncation or migration.
> 
> Is there anything that prevents us from holding a reference on a page
> under writeback?

Hm, I'm hesitant to add redundant life-time management to the page
there just for memcg, which is not always configured in.

Pinning the memcg instead is slightly more complex, but IMO has the
complexity in a preferrable place.

Would you agree?