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

[Dev Jain]

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 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;
>>>
>>> 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...

>