Re: [patch 2/2] mm: memcontrol: fix missed end-writeback page accounting

Johannes Weiner
2014-10-23T14:14:43Z

On Thu, Oct 23, 2014 at 03:03:31PM +0200, Michal Hocko wrote:
> On Wed 22-10-14 14:29:28, Johannes Weiner wrote:
> > 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") changed page
> > migration to uncharge the old page right away. The page is locked,
> > unmapped, truncated, and off the LRU, but it could race with writeback
> > ending, which then doesn't unaccount the page properly:
> >
> > test_clear_page_writeback() migration
> > acquire pc->mem_cgroup->move_lock
>
> I do not think that mentioning move_lock is important/helpful here
> because the hot path which is taken all the time (except when there is a
> task move in progress) doesn't take it.
> Besides that it is not even relevant for the race.

You're right. It's not worth mentioning the transaction setup/finish
at all, because migration does not participate in that protocol. How
about this? Andrew, could you please copy-paste this into the patch?

test_clear_page_writeback() migration
wait_on_page_writeback()
TestClearPageWriteback()
mem_cgroup_migrate()
clear PCG_USED
mem_cgroup_update_page_stat()
if (PageCgroupUsed(pc))
decrease memcg pages under writeback