On Wed 30-10-19 15:33:07, 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: [...] > > > 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(). > > 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. Thanks for the clarification! This sounds reasonable (as much as the mlock juggling does) to me. This is probably worth a comment right above the bug_on. I have to confess that I am still not clear on all the details here, though. E.g. migrate_misplaced_transhuge_page sets the flag without lru_lock and relies only on page lock IIUC and the bug on is done right after the lock is released. Maybe I am just confused or maybe the race window is too small to matter but isn't this race possible at least theoretically? -- Michal Hocko SUSE Labs
