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SubjectRe: [Patch v3] mm: thp: grab the lock before manipulation defer list
On Fri, 17 Jan 2020, Kirill Tkhai wrote:

> >> I think that's a good point, especially considering that the current code
> >> appears to unconditionally place any compound page on the deferred split
> >> queue of the destination memcg. The correct list that it should appear
> >> on, I believe, depends on whether the pmd has been split for the process
> >> being moved: note the MC_TARGET_PAGE caveat in
> >> mem_cgroup_move_charge_pte_range() that does not move the charge for
> >> compound pages with split pmds. So when mem_cgroup_move_account() is
> >> called with compound == true, we're moving the charge of the entire
> >> compound page: why would it appear on that memcg's deferred split queue?
> >
> > I believe Kirill asked how do we know that the page should be actually
> > added to the deferred list just from the list_empty check. In other
> > words what if the page hasn't been split at all?
>
> Yes, I'm talking about this. Function mem_cgroup_move_account() adds every
> huge page to the deferred list, while we need to do that only for pages,
> which are queued for splitting...
>

Yup, and that appears broken before Wei's patch. Since we only migrate
charges of entire compound pages (we have a mapping pmd, the underlying
page cannot be split), it should not appear on the deferred split queue
for any memcg, right?

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