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

[Michal Hocko]

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.

> 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?
-- 
Michal Hocko
SUSE Labs