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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/3/25 21:09, Gregory Price wrote:
> 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).

Not sure I understand correctly. I think the fix from Mel was the right 
thing to do.

It does not make sense to try migrating a 1GB page when allocating a 1GB 
page. Ever.

-- 
Cheers

David