Re: [patch] mm: memcontrol: update mem_cgroup_page_lruvec() documentation

[Johannes Weiner]

On Mon, Oct 20, 2014 at 09:12:56PM +0200, Michal Hocko wrote:
> On Sun 19-10-14 11:30:16, Johannes Weiner wrote:
> > 7512102cf64d ("memcg: fix GPF when cgroup removal races with last
> > exit") added a pc->mem_cgroup reset into mem_cgroup_page_lruvec() to
> > prevent a crash where an anon page gets uncharged on unmap, the memcg
> > is released, and then the final LRU isolation on free dereferences the
> > stale pc->mem_cgroup pointer.
> > 
> > But since 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API"), pages
> > are only uncharged AFTER that final LRU isolation, which guarantees
> > the memcg's lifetime until then.  pc->mem_cgroup now only needs to be
> > reset for swapcache readahead pages.
> 
> Do we want VM_BUG_ON_PAGE(!PageSwapCache, page) into the fixup path?

While that is what we expect as of right now, it's not really a
requirement for this function.  Should somebody later add other page
types they might trigger this assertion and scratch their head about
it and wonder if they're missing some non-obvious dependency.

> > Update the comment and callsite requirements accordingly.
> > 
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> Acked-by: Michal Hocko <mhocko@xxxxxxx>

Thanks!
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