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 8/13/24 12:52, Dev Jain wrote:
>>
>> On 8/13/24 10:30, Dev Jain wrote:
>>>
>>> On 8/12/24 17:38, Dev Jain wrote:
>>>>
>>>> On 8/12/24 13:01, Huang, Ying wrote:
>>>>> Dev Jain <dev.jain@arm.com> writes:
>>>>>
>>>>>> On 8/12/24 11:45, Huang, Ying wrote:
>>>>>>> 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.
>>>>>> Yes, that's my point. We need to undo src+dst and retry.
>>>>> For sync migration, we undo src+dst and retry now, but only once.  You
>>>>> have shown that more retrying increases success rate.
>>>>>
>>>>>> We will have
>>>>>> to decide where we want this retrying to be; do we want to change the
>>>>>> return value, end up in the while loop wrapped around _sync(),
>>>>>> and retry
>>>>>> there by adding another level of loop, or do we want to make use
>>>>>> of the
>>>>>> existing retry loops, one of which is wrapped around _unmap();
>>>>>> the latter
>>>>>> is my approach. The utility I see for the former approach is
>>>>>> that, in case
>>>>>> of a large number of page migrations (which should usually be
>>>>>> the case),
>>>>>> we are giving more time for the folio to get retried. The latter
>>>>>> does not
>>>>>> give much time and discards the folio if it did not succeed
>>>>>> under 7 times.
>>>>> Because it's a race, I guess that most folios will be migrated
>>>>> successfully in the first pass.
>>>>>
>>>>> My concerns of your method are that it deal with just one case
>>>>> specially.  While retrying after undoing all appears more general.
>>>>
>>>>
>>>> Makes sense. Also, please ignore my "change the return value"
>>>> thing, I got confused between unmap_folios, ret_folios, etc.
>>>> Now I think I understood what the lists are doing :)
>>>>
>>>>>
>>>>> If it's really important to retry after undoing all, we can either
>>>>> convert two retying loops of migrate_pages_batch() into one loop, or
>>>>> remove retry loop in migrate_pages_batch() and retry in its caller
>>>>> instead.
>>>>
>>>> And if I implemented this correctly, the following makes the test
>>>> pass always:
>>>> https://www.codedump.xyz/diff/Zrn7EdxzNXmXyNXe
>>>
>>>
>>> Okay, I did mess up with the implementation, leading to a false
>>> positive. Let me try again :)
>>
>>
>> Hopefully this should do the job:
>> https://www.codedump.xyz/diff/ZrsIV8JSOPYx5V_u
>>
>> But the result is worse than the patch proposed; I rarely hit
>> a 3 digit number of successes of move_pages(). But, on a
>> base kernel without any changes, when I apply David's
>> suggestion to change the test, if I choose 7 as the number
>> of retries (= NR_MAX_MIGRATE_SYNC_RETRY) in the test, I
>> can touch even 4 digits. I am puzzled.
>> We can also try merging the for loops of unmap and move...
>
>
> If people are okay with this change, I guess I can send it as
> a v2? I concur with your assessment that my initial approach
> is solving a specific case; the above approach does give me
> a slight improvement on arm64 and should be an improvement
> in general, since it makes sense to defer retrying the failed folio
> as much as we can.

We need to deal with something else before a formal v2,

- stats need to be fixed, please check result processing for the first
  loop of migrate_pages_sync().

- Do we need something similar for async migration.

- Can we add another level of explicit loop for the second loop of
  migrate_pages_sync()?  That is to improve code readability.  Or, add a
  function to dot that?

- Is it good to remove retry loop in migrate_pages_batch()?  And do
  retry in the caller?

--
Best Regards,
Huang, Ying