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

[Dev Jain]

On 8/12/24 11:04, Huang, Ying wrote:
> Hi, Dev,
>
> Dev Jain <dev.jain at arm.com> writes:
>
>> 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 at 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;
>>   		__migrate_folio_record(dst, old_page_state, anon_vma);
>>   		return MIGRATEPAGE_UNMAP;
>>   	}
> Do you have some test results for this?  For example, after applying the
> patch, the migration success rate increased XX%, etc.

Noting that the migration selftest is operating on a single page,
before the patch, the test fails on shared-anon mappings on an
average of 10 iterations of move_pages(), and after applying the
patch it fails on average of 100 iterations, which makes sense
because the unmapping() will get retried 3 + 7 = 10 times.

>
> My understanding for this issue is that the migration success rate can
> increase if we undo all changes before retrying.  This is the current
> behavior for sync migration, but not for async migration.  If so, we can
> use migrate_pages_sync() for async migration too to increase success
> rate?  Of course, we need to change the function name and comments.
>
> --
> Best Regards,
> Huang, Ying