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? I don't think it's reasonable to pass unevictalbe LRU pages into shrink_page_list so wanted to know what race is here what we are missing to remove the BUG_ON since mlock is heavily complicated. > > -- > Michal Hocko > SUSE Labs
