Re: [PATCH] mm: mempolicy: handle vma with unmovable pages mapped correctly in mbind

From: Michal Hocko
Date: Wed Jun 19 2019 - 01:26:45 EST

On Tue 18-06-19 14:13:16, Yang Shi wrote:
> > > > > Change migrate_page_add() to check if the page is movable or not, if it
> > > > > is unmovable, just return -EIO. We don't have to check non-LRU movable
> > > > > pages since just zsmalloc and virtio-baloon support this. And, they
> > > > > should be not able to reach here.
> > > > You are not checking whether the page is movable, right? You only rely
> > > > on PageLRU check which is not really an equivalent thing. There are
> > > > movable pages which are not LRU and also pages might be off LRU
> > > > temporarily for many reasons so this could lead to false positives.
> > > I'm supposed non-LRU movable pages could not reach here. Since most of them
> > > are not mmapable, i.e. virtio-balloon, zsmalloc. zram device is mmapable,
> > > but the page fault to that vma would end up allocating user space pages
> > > which are on LRU. If I miss something please let me know.
> > That might be true right now but it is a very subtle assumption that
> > might break easily in the future. The point is still that even LRU pages
> > might be isolated from the LRU list temporarily and you do not want this
> > to cause the failure easily.
> I used to have !__PageMovable(page), but it was removed since the
> aforementioned reason. I could add it back.
> For the temporary off LRU page, I did a quick search, it looks the most
> paths have to acquire mmap_sem, so it can't race with us here. Page
> reclaim/compaction looks like the only race. But, since the mapping should
> be preserved even though the page is off LRU temporarily unless the page is
> reclaimed, so we should be able to exclude temporary off LRU pages by
> calling page_mapping() and page_anon_vma().
> So, the fix may look like:
> if (!PageLRU(head) && !__PageMovable(page)) {
>     if (!(page_mapping(page) || page_anon_vma(page)))
>         return -EIO;

This is getting even more muddy TBH. Is there any reason that we have to
handle this problem during the isolation phase rather the migration?
Michal Hocko