Re: [PATCH] mm: migrate: fix an incorrect call of prep_transhuge_page()

[Michal Hocko]

On Wed 22-11-17 07:29:38, Zi Yan wrote:
> On 22 Nov 2017, at 7:13, Zi Yan wrote:
> 
> > On 22 Nov 2017, at 5:14, Michal Hocko wrote:
> >
> >> On Wed 22-11-17 10:35:10, Michal Hocko wrote:
> >> [...]
> >>> Moreover I am not really sure this is really working properly. Just look
> >>> at the split_huge_page. It moves all the tail pages to the LRU list
> >>> while migrate_pages has a list of pages to migrate. So we will migrate
> >>> the head page and all the rest will get back to the LRU list. What
> >>> guarantees that they will get migrated as well.
> >>
> >> OK, so this is as I've expected. It doesn't work! Some pfn walker based
> >> migration will just skip tail pages see madvise_inject_error.
> >> __alloc_contig_migrate_range will simply fail on THP page see
> >> isolate_migratepages_block so we even do not try to migrate it.
> >> do_move_page_to_node_array will simply migrate head and do not care
> >> about tail pages. do_mbind splits the page and then fall back to pte
> >> walk when thp migration is not supported but it doesn't handle tail
> >> pages if the THP migration path is not able to allocate a fresh THP
> >> AFAICS. Memory hotplug should be safe because it doesn't skip the whole
> >> THP when doing pfn walk.
> >>
> >> Unless I am missing something here this looks like a huge mess to me.
> >
> > +Kirill
> >
> > First, I agree with you that splitting a THP and only migrating its head page
> > is a mess. But what you describe is also the behavior of migrate_page()
> > _before_ THP migration support is added. I thought that was intended.
> >
> > Look at http://elixir.free-electrons.com/linux/v4.13.15/source/mm/migrate.c#L1091,
> > unmap_and_move() splits THPs and only migrates the head page in v4.13 before THP
> > migration is added. I think the behavior was introduced since v4.5 (I just skimmed
> > v4.0 to v4.13 code and did not have time to use git blame), before that THPs are
> > not migrated but shown as successfully migrated (at least from v4.4âs code).
> 
> Sorry, I misread v4.4âs code, it also does âsplitting a THP and migrating its head pageâ.
> This behavior was there for a long time, at least since v3.0.
> 
> The code in unmap_and_move() is:
> 
> if (unlikely(PageTransHuge(page)))
> 		if (unlikely(split_huge_page(page)))
> 			goto out;

I _think_ that this all should be handled at migrate_pages layer. Try to
migrate THP and fallback to split_huge_page into to the list when it
fails. I haven't checked whether there is something which would prevent
that though. THP tricks in specific paths then should be removed.


-- 
Michal Hocko
SUSE Labs