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

[Zi Yan]

On 3 Nov 2026, at 10:12, Vlastimil Babka (SUSE) wrote:

> 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@xxxxxxxxxxxxxxx
>> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
>
> I've applied this single patch to page_alloc/for-next-fixes and will drop
> the version from Josef [1]. Also transferred Zi's R-b since it's the same
> code change (plus comment).
>
> [1]
> https://lore.kernel.org/all/20261027-pgalloc-ai-hotfixes-v1-1-9c7d1aca801e@xxxxxxxxxx/

Thanks.

Best Regards,
Yan, Zi