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

[Yang Shi]

On 6/19/19 1:22 AM, Vlastimil Babka wrote:
> On 6/19/19 7:21 AM, Michal Hocko wrote:
> > On Tue 18-06-19 14:13:16, Yang Shi wrote:
> > [...]
> > > 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)) {
> > >  ÂÂÂ if (!(page_mapping(page) || page_anon_vma(page)))
> > >  ÂÂÂ ÂÂÂ 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?
> I think it was already said that if pages can't be isolated, then
> migration phase won't process them, so they're just ignored.

Yesïexactly.

> However I think the patch is wrong to abort immediately when
> encountering such page that cannot be isolated (AFAICS). IMHO it should
> still try to migrate everything it can, and only then return -EIO.

It is fine too. I don't see mbind semantics define how to handle such 
case other than returning -EIO.