Re: [RFC PATCH 2/2] mm,page_alloc: Make alloc_contig_range handle free hugetlb pages

[David Hildenbrand]

On 10.02.21 15:24, Oscar Salvador wrote:
> On Wed, Feb 10, 2021 at 09:23:59AM +0100, David Hildenbrand wrote:
> > On 08.02.21 11:38, Oscar Salvador wrote:
> > > Free hugetlb pages are trickier to handle as to in order to guarantee
> > > no userspace appplication 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
> > > in introduced.
> > > This function will first try to get a new fresh hugetlb page, and if it
> > > succeeds, it will dissolve the old one.
> >
> > Thanks for looking into this! Can we move this patch to #1 in the series? It
> > is the easier case.
> >
> > I also wonder if we should at least try on the memory unplug path to keep
> > nr_pages by at least trying to allocate at new one if required, and printing
> > a warning if that fails (after all, we're messing with something configured
> > by the admin - "nr_pages"). Note that gigantic pages are special (below).
>
> So, do you mean to allocate a new fresh hugepage in case we have a free
> hugetlb page within the range we are trying to offline? That makes some
> sense I guess.
>
> I can have a look at that, and make hotplug code use the new
> alloc_and_dissolve().

Yes, with the difference that hotplug code most probably wants to 
continue even if allocation failed (printing a warning) - mimix existing 
behavior. For alloc_contig, I'd say, fail if we cannot "relocate free 
huge pages that are still required to no modify nr_pages".

alloc_and_dissolve() should only allocate a page if really required 
(e.g., not sure if we could skip allocation in some cases - like with 
surplus pages, needs some investigation), such that the admin-configured 
nr_pages stays unchanged.

--
Thanks,

David / dhildenb