On Mon, Oct 20, 2014 at 09:12:56PM +0200, Michal Hocko wrote:

On Sun 19-10-14 11:30:16, Johannes Weiner wrote:

quoted

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.

quoted

Update the comment and callsite requirements accordingly.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <redacted>

Thanks!

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