Re: [PATCH v4] page_alloc: allow migration of smaller hugepages during contig_alloc

Gregory Price
2025-12-03T20:09:59Z

On Wed, Dec 03, 2025 at 08:43:29PM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/3/25 19:01, Frank van der Linden wrote:
> >
> > The PageHuge() check seems a bit out of place there, if you just
> > removed it altogether you'd get the same results, right? The isolation
> > code will deal with it. But sure, it does potentially avoid doing some
> > unnecessary work.
>
> commit 4d73ba5fa710fe7d432e0b271e6fecd252aef66e
> Author: Mel Gorman <mgorman@techsingularity.net>
> Date: Fri Apr 14 15:14:29 2023 +0100
>
> mm: page_alloc: skip regions with hugetlbfs pages when allocating 1G pages
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
> taking an excessive amount of time for large amounts of memory. Further
> testing allocating huge pages that the cost is linear i.e. if allocating
> 1G pages in batches of 10 then the time to allocate nr_hugepages from
> 10->20->30->etc increases linearly even though 10 pages are allocated at
> each step. Profiles indicated that much of the time is spent checking the
> validity within already existing huge pages and then attempting a
> migration that fails after isolating the range, draining pages and a whole
> lot of other useless work.
> Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig") removed two checks, one which ignored huge pages
> for contiguous allocations as huge pages can sometimes migrate. While
> there may be value on migrating a 2M page to satisfy a 1G allocation, it's
> potentially expensive if the 1G allocation fails and it's pointless to try
> moving a 1G page for a new 1G allocation or scan the tail pages for valid
> PFNs.
> Reintroduce the PageHuge check and assume any contiguous region with
> hugetlbfs pages is unsuitable for a new 1G allocation.
>

Worth noting that because this check really only applies to gigantic
page *reservation* (not faulting), this isn't necessarily incurred in a
time critical path. So, maybe i'm biased here, the reliability increase
feels like a win even if the operation can take a very long time under
memory pressure scenarios (which seems like an outliar anyway).

~Gregory