Re: [PATCH 1/2] mm: Retry migration earlier upon refcount mismatch

[Zi Yan <ziy@nvidia.com>]

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On 9 Aug 2024, at 9:47, David Hildenbrand wrote:

> On 09.08.24 12:31, Dev Jain wrote:
>> As already being done in __migrate_folio(), wherein we backoff if the
>> folio refcount is wrong, make this check during the unmapping phase, upon
>> the failure of which, the original state of the PTEs will be restored and
>> the folio lock will be dropped via migrate_folio_undo_src(), any racing
>> thread will make progress and migration will be retried.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   mm/migrate.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index e7296c0fb5d5..477acf996951 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>>   	}
>>    	if (!folio_mapped(src)) {
>> +		/*
>> +		 * Someone may have changed the refcount and maybe sleeping
>> +		 * on the folio lock. In case of refcount mismatch, bail out,
>> +		 * let the system make progress and retry.
>> +		 */
>> +		struct address_space *mapping = folio_mapping(src);
>> +
>> +		if (folio_ref_count(src) != folio_expected_refs(mapping, src))
>> +			goto out;
>
> This really seems to be the latest point where we can "easily" back off and unlock the source folio -- in this function :)
>
> I wonder if we should be smarter in the migrate_pages_batch() loop when we start the actual migrations via migrate_folio_move(): if we detect that a folio has unexpected references *and* it has waiters (PG_waiters), back off then and retry the folio later. If it only has unexpected references, just keep retrying: no waiters -> nobody is waiting for the lock to make progress.
>
> For example, when migrate_folio_move() fails with -EAGAIN, check if there are waiters (PG_waiter?) and undo+unlock to try again later.
>
> But I'm not really a migration expert, so only my 2 cents :)

Sounds reasonable to me. Also add Ying (the author of batch page migration).

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Best Regards,
Yan, Zi

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