On 2019/10/31 3:33, Johannes Weiner wrote: > On Wed, Oct 30, 2019 at 06:45:33PM +0100, Michal Hocko wrote: >> On Wed 30-10-19 09:52:39, Minchan Kim wrote: >>> On Tue, Oct 29, 2019 at 06:45:12PM +0800, zhong jiang wrote: >>>> On 2019/10/29 17:40, Michal Hocko wrote: >>>>> On Tue 29-10-19 17:30:57, zhong jiang wrote: >>>>>> On 2019/10/29 16:11, Michal Hocko wrote: >>>>>>> [Cc Minchan] >>>>> [...] >>>>>>> Removing a long existing BUG_ON begs for a much better explanation. >>>>>>> shrink_page_list is not a trivial piece of code but I _suspect_ that >>>>>>> removing it should be ok for mapped pages at least (try_to_unmap) but I >>>>>>> am not so sure how unmapped unevictable pages are handled from top of my >>>>>>> head. >>>>>> As to the unmapped unevictable pages. shrink_page_list has taken that into account. >>>>>> >>>>>> shinkr_page_list >>>>>> page_evictable --> will filter the unevictable pages to putback its lru. >>>>> Ohh, it is right there at the top. Missed it. The check has been added >>>>> by Nick along with the BUG_ON. So it is sounds more like a "this >>>>> shouldn't happen" bugon. I wouldn't mind to remove it with that >>>>> justification. >>>> As you has said, Minchan fix the same kind of bug by checking PageUnevictable (I did not notice before) >>>> Wait for Minchan to see whether he has better reason. thanks, >>> madvise_pageout could work with a shared page and one of the vmas among processes >>> could do mlock so it could pass Unevictable LRU pages into shrink_page_list. >>> It's pointless to try reclaim unevictable pages from the beginning so I want to fix >>> madvise_pageout via introducing only_evictable flag into the API so that >>> madvise_pageout uses it as "true". >>> >>> If we want to remove the PageUnevictable VM_BUG_ON_PAGE in shrink_page_list, >>> I want to see more strong reason why it happens and why caller couldn't >>> filter them out from the beginning. >> Why is this preferable over removing the VM_BUG_ON condition? In other >> words why should we keep PageUnevictable check there? > The mlock LRU shuffling is a bit tricky and can race with page reclaim > or others isolating the page from the LRU list. If another isolator > wins, it has to move the page during putback on behalf of mlock. > > See the implementation and comments in __pagevec_lru_add_fn(). I see that comments in __pagevec_lru_add_fn. I have some confusion. It will result in evictable page strand in an unevictable lru without PageMlocked due to disorder If I understand it correctly. vmscan can see !page_evictable(). It should be PageMLocked is set in evictable list. Is there any race window ? Thanks, zhong jiang > That's why page reclaim can see !page_evictable(), but it must not see > pages that have the PageUnevictable lru bit already set. Because that > would mean the isolation/putback machinery messed up somewhere and the > page LRU state is corrupt. > > As that machinery is non-trivial, it's useful to have that sanity > check in page reclaim. > > . >
