Re: [PATCH] mm: fix unevictable page reclaim when calling madvise_pageout

[Minchan Kim <minchan@kernel.org>]

On Thu, Oct 31, 2019 at 10:16:01AM +0100, Michal Hocko wrote:
> 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?

IIUC, reclaim couldn't see the page from LRU list because it was isolated by
numamigrate_isolate_page.

Thanks.