Re: [PATCH 1/2] mm: Make alloc_contig_range handle free hugetlb pages

[David Hildenbrand]

On 17.02.21 14:30, Michal Hocko wrote:
> On Wed 17-02-21 11:08:15, Oscar Salvador wrote:
> > Free hugetlb pages are tricky to handle so as to no userspace application
> > notices disruption, we need to replace the current free hugepage with
> > a new one.
> >
> > In order to do that, a new function called alloc_and_dissolve_huge_page
> > is introduced.
> > This function will first try to get a new fresh hugetlb page, and if it
> > succeeds, it will dissolve the old one.
> >
> > With regard to the allocation, since we do not know whether the old page
> > was allocated on a specific node on request, the node the old page belongs
> > to will be tried first, and then we will fallback to all nodes containing
> > memory (N_MEMORY).
>
> I do not think fallback to a different zone is ok. If yes then this
> really requires a very good reasoning. alloc_contig_range is an
> optimistic allocation interface at best and it shouldn't break carefully
> node aware preallocation done by administrator.

What does memory offlining do when migrating in-use hugetlbfs pages? 
Does it always keep the node?

I think keeping the node is the easiest/simplest approach for now.

> > Note that gigantic hugetlb pages are fenced off since there is a cyclic
> > dependency between them and alloc_contig_range.
>
> Why do we need/want to do all this in the first place?

cma and virtio-mem (especially on ZONE_MOVABLE) really want to handle 
hugetlbfs pages.

--
Thanks,

David / dhildenb