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

["Huang, Ying" <ying.huang@intel.com>]

Dev Jain <dev.jain@arm.com> writes:

> On 8/12/24 11:04, Huang, Ying wrote:
>> Hi, Dev,
>>
>> Dev Jain <dev.jain@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@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.
>
> I'll get back to you on this.
>
>>
>> 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.
>
>
> As per my understanding, this is not the current behaviour for sync
> migration. After successful unmapping, we fail in migrate_folio_move()
> with -EAGAIN, we do not call undo src+dst (rendering the loop around
> migrate_folio_move() futile), we do not push the failed folio onto the
> ret_folios list, therefore, in _sync(), _batch() is never tried again.

In migrate_pages_sync(), migrate_pages_batch(,MIGRATE_ASYNC) will be
called first, if failed, the folio will be restored to the original
state (unlocked).  Then migrate_pages_batch(,_SYNC*) is called again.
So, we unlock once.  If it's necessary, we can unlock more times via
another level of loop.

--
Best Regards,
Huang, Ying