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

[Dev Jain <dev.jain@arm.com>]

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...

>
>>
>>
>>
>>>
>>> -- 
>>> Best Regards,
>>> Huang, Ying
>>
>