Re: [PATCH 4/6] mm/page_alloc: don't undo another isolation in alloc_contig_range

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

On 10/27/26 05:22, Gregory Price (Meta) wrote:
> An alloc_contig_range() that overlaps a block going offline can leave
> the offline spinning until it gets a signal, holding the device hotplug
> lock.  When isolation fails, alloc_contig_frozen_range() undoes it over
> the whole range, clearing the offline's MIGRATE_ISOLATE too:
> 
>   CPU 1 (offline_pages)               CPU 2 (alloc_contig_range)
>   start_isolate_page_range()
>     blocks are MIGRATE_ISOLATE
>                                       start_isolate_page_range()
>                                         -EBUSY on CPU 1's blocks
>                                       undo_isolate_page_range()
>                                         blocks are MIGRATE_MOVABLE
>   test_pages_isolated()
>     never succeeds again
> 
> start_isolate_page_range() fails only on a block someone else isolated,
> after undoing its own, so this undo only ever clears another caller's.
> Return directly, as before alloc_contig_range() went pageblock granular.
> 
> With offline_pages() parked after isolating a DIMM block and a 1G
> hugetlb allocation made over it, the offline on base still loops 30s
> later in 4 of 4 runs:
> 
>   offline_pages+0x5b2/0x1570
>   memory_subsys_offline+0x17e/0x470
>   device_offline+0x250/0x340
>   state_store+0x88/0x110
> 
> With this patch the allocation still fails, and the offline completes in
> 3 of 3 runs.
> 
> Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
> ---

We have an existing AI fix for that as well already handled by Vlasta.

I think we'll have to establish some model of "ask David first before posting"
because otherwise we just create duplicate work for everyone.

-- 
Cheers,

David