Re: [PATCH v3] mm, hugetlb: implement movable_gigantic_pages sysctl

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

On 12/4/25 18:14, Gregory Price wrote:
> On Wed, Dec 03, 2025 at 10:26:20AM +0100, David Hildenbrand (Red Hat) wrote:
>> On 12/3/25 07:38, Gregory Price wrote:
>>> This reintroduces a concept removed by:
>>> commit d6cb41cc44c6 ("mm, hugetlb: remove hugepages_treat_as_movable sysctl")
>>>
>>> This sysctl provides flexibility between ZONE_MOVABLE use cases:
>>> 1) onlining memory in ZONE_MOVABLE to maintain hotplug compatibility
>>> 2) onlining memory in ZONE_MOVABLE to make hugepage allocate reliable
>>>
>>> When ZONE_MOVABLE is used to make huge page allocation more reliable,
>>> disallowing gigantic pages memory in this region is pointless.  If
>>> hotplug is not a requirement, we can loosen the restrictions to allow
>>> 1GB gigantic pages in ZONE_MOVABLE.
>>>
>>> Since 1GB can be difficult to migrate / has impacts on compaction /
>>> defragmentation, we don't enable this by default. Notably, 1GB pages
>>> can only be migrated if another 1GB page is available - so hot-unplug
>>> will fail if such a page cannot be found.
>>
>> In light of the other discussion: will it fail or will it simplt retry
>> forever, until there is a free 1g page?
>>
> 
> It retries until a 1GB page is available.
> 
> Example test:
> 
> echo 0 > node0/hugepages/..-1GB/nr_hugepages (dram node)
> echo 1 > node1/hugepages/..-1GB/nr_hugepages (zone_movable node)
> ./alloc_huge &                               (allocate the page)
> ./node1_offline  &                           (offline > memory*/state)
> sleep 5                                      (give offline time)
> echo 1 > node0/hugepages/..-1GB/nr_hugepages (dram node)
> 
> This node1_offline generates migration failures until the last step
> occurs, at which point migration and node1_offline complete as expected.
> 
> The migration failures produce the following:
> 
> [  707.443105] migrating pfn c080000 failed ret:-12
> [  707.453353] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0xc080000
> [  707.471315] head: order:18 mapcount:1 entire_mapcount:1 nr_pages_mapped:0 pincount:0
> [  707.488504] anon flags: 0x17ffff0000000848(uptodate|owner_2|head|node=1|zone=3|lastcpupid=0x1ffff)
> [  707.508393] page_type: f4(hugetlb)
> [  707.515940] raw: 17ffff0000000848 ffa000007d873cc0 ffa000007d873cc0 ff1100082366c6e9
> [  707.533126] raw: 0000000000000000 0000000000000010 00000002f4000000 0000000000000000
> [  707.550317] head: 17ffff0000000848 ffa000007d873cc0 ffa000007d873cc0 ff1100082366c6e9
> [  707.567699] head: 0000000000000000 0000000000000010 00000002f4000000 0000000000000000
> [  707.585085] head: 17ffff0000000012 ffd4000302000001 0000000000000000 0000000000000000
> [  707.602469] head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000040000
> [  707.619851] page dumped because: migration failure
> 
> 
> I can add this to the changelog if you prefer

Yes, we should document that. I guess it's just what we already document 
in the memory hotplug doc: it keeps retrying until there is sufficient 
free memory.


-- 
Cheers

David