On 17.01.2020 12:32, David Rientjes wrote: > 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? Hm. Can't a huge page be mapped in two tasks: 1)the first task unmapped a part of page and initiated splitting, 2)the second task still refers the whole page, then we move account for the second task?
