Re: [RFC PATCH 0/5] mm, hugetlb: allocation API and migration improvements

From: Michal Hocko
Date: Thu Dec 21 2017 - 02:28:12 EST

On Wed 20-12-17 14:43:03, Mike Kravetz wrote:
> On 12/20/2017 01:53 AM, Michal Hocko wrote:
> > On Wed 20-12-17 05:33:36, Naoya Horiguchi wrote:
> >> I have one comment on the code path from mbind(2).
> >> The callback passed to migrate_pages() in do_mbind() (i.e. new_page())
> >> calls alloc_huge_page_noerr() which currently doesn't call SetPageHugeTemporary(),
> >> so hugetlb migration fails when h->surplus_huge_page >= h->nr_overcommit_huge_pages.
> >
> > Yes, I am aware of that. I should have been more explicit in the
> > changelog. Sorry about that and thanks for pointing it out explicitly.
> > To be honest I wasn't really sure what to do about this. The code path
> > is really complex and it made my head spin. I fail to see why we have to
> > call alloc_huge_page and mess with reservations at all.
> Oops! I missed that in my review.
> Since alloc_huge_page was called with avoid_reserve == 1, it should not
> do anything with reserve counts. One potential issue with the existing
> code is cgroup accounting done by alloc_huge_page. When the new target
> page is allocated, it is charged against the cgroup even though the original
> page is still accounted for. If we are 'at the cgroup limit', the migration
> may fail because of this.

Yeah, the existing code seems just broken. I strongly suspect that the
allocation API for hugetlb was so complicated that this was just a
natural result of a confusion with some follow up changes on top.

> I like your new code below as it explicitly takes reserve and cgroup
> accounting out of the picture for migration. Let me think about it
> for another day before providing a Reviewed-by.

Thanks a lot!
Michal Hocko