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

[David Hildenbrand <david@redhat.com>]

On 11.08.24 08:06, Dev Jain wrote:
> 
> On 8/11/24 00:22, David Hildenbrand wrote:
>> On 10.08.24 20:42, Dev Jain wrote:
>>>
>>> On 8/9/24 19:17, 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.
>>>
>>>
>>> The patch currently retries migration irrespective of the reason of
>>> refcount change.
>>>
>>> If you are suggesting that, break the retrying according to two
>>> conditions:
>>
>> That's not what I am suggesting ...
>>
>>>
>>>
>>>> This really seems to be the latest point where we can "easily" back
>>>> off and unlock the source folio -- in this function :)
>>>> For example, when migrate_folio_move() fails with -EAGAIN, check if
>>>> there are waiters (PG_waiter?) and undo+unlock to try again later.
>>>
>>>
>>> Currently, on -EAGAIN, migrate_folio_move() returns without undoing src
>>> and dst; even if we were to fall
>>
>> ...
>>
>> I am wondering if we should detect here if there are waiters and undo
>> src+dst.
> 
> After undoing src+dst, which restores the PTEs, how are you going to set the
> 
> PTEs to migration again? That is being done through migrate_folio_unmap(),
> 
> and the loops of _unmap() and _move() are different. Or am I missing
> something...

Again, no expert on the code, but it would mean that if we detect that 
there are waiters, we would undo src+dst and add them to ret_folios, 
similar to what we do in "Cleanup remaining folios" at the end of 
migrate_pages_batch()?

So instead of retrying migration of that folio, just give it up 
immediately and retry again later.

Of course, this means that (without further modifications to that 
function), we would leave retrying these folios to the caller, such as 
in migrate_pages_sync(), where we move ret_folios to the tail of 
"folios" and retry migration.

Maybe one would want to optimize that retry logic with such "temporarily 
failed because someone else has to make progress for us to make progress 
and free up a page reference" case. These are different to the typical 
"speculative" references that we try to handle via the existing retry magic.

Please let me know if I am missing something fundamental.

-- 
Cheers,

David / dhildenb