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

[David Hildenbrand]

On 17.02.21 14:50, Michal Hocko wrote:
> On Wed 17-02-21 14:36:47, David Hildenbrand wrote:
> > 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?
>
> No it will break the node pool. The reasoning behind that is that
> offlining is an explicit request from the userspace and it is expected

userspace? in 99,9996% it's the hardware that triggers the unplug of a DIMM.

> > 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.
>
> Do we have any real life examples? Or does this fall more into, let's
> optimize an existing implementation category.

It's a big TODO item I have on my list and I am happy that Oscar is 
looking into it. So yes, I noticed it while working on virtio-mem. It's 
real.

--
Thanks,

David / dhildenb