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?