On 2019/11/6 2:48, Johannes Weiner wrote: > On Tue, Nov 05, 2019 at 10:10:37PM +0800, zhong jiang wrote: >> On 2019/11/5 20:45, Michal Hocko wrote: >>> On Tue 05-11-19 20:28:58, zhong jiang wrote: >>>> On 2019/11/5 14:33, Michal Hocko wrote: >>>>> On Sat 02-11-19 15:36:55, zhong jiang wrote: >>>>>> On 2019/11/2 2:32, Michal Hocko wrote: >>>>> [...] >>>>>>> But I would really appreciate to add a comment for the BUG_ON and >>>>>>> explain why do we care about PageUnevictable so much when there is an >>>>>>> explicit page_evictable check in the reclaim path. In other words a >>>>>>> short summary of what Johannes explained in >>>>>>> http://lkml.kernel.org/r/20191030193307.GA48128@xxxxxxxxxxx. Maybe in a >>>>>>> separate patch. Care to send one or should I send it? >>>>>> Hi, Michal >>>>>> >>>>>> Actually, I am not very clear about the words Johannes had said. How the race to >>>>>> tirgger, it will result in an PgeMlocked page can be visible in shrink_page_list. >>>>>> >>>>>> Could you elaborate the race in detail further ? >>>>> I would go with the following comment >>>>> >>>>> /* >>>>> * Page reclaim can see !page_evictable(), but it must not see pages that >>>>> * have the PageUnevictable lru bit already set. See __pagevec_lru_add_fn() >>>>> * for more details. >>>>> */ >>>> But the detail still confuses me in __pagevec_lru_add_fn() to see PageMlocked in vmscan :-\ . >>> Which part does confuse you exactly? >> page reclaim can see !page_evictable() means some race still exist in the kernel. Is there any race window . > Yes. mlock does this: > > lock_page() > SetPageMlocked() > if (isolate_lru_page()) > putback_lru_page() // move to unevictable list > unlock_page() > > and vmscan does this: > > isolate_lru_pages() > for_each_page() > if (!try_lock_page()) > continue > if (!page_evictable()) > continue > putback_lru_pages() > > It's possible that mlock locks the page and sets PG_mlocked, but > vmscan has the page already isolated and mlock cannot move it to the > unevictable list itself. In that case, vmscan will either fail to lock > the page or see !page_unevictable() and move the page on the > unevictable list on behalf of mlock. Thanks for you kindly clarification. Sincerely, zhong jiang > . >
