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

[Dev Jain]

On 8/12/24 11:50, Huang, Ying wrote:
> Dev Jain <dev.jain at arm.com> writes:
>
>> On 8/12/24 11:04, Huang, Ying wrote:
>>> Hi, Dev,
>>>
>>> Dev Jain <dev.jain at 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 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;
>>>>    		__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.
>> Noting that the migration selftest is operating on a single page,
>> before the patch, the test fails on shared-anon mappings on an
>> average of 10 iterations of move_pages(), and after applying the
>> patch it fails on average of 100 iterations, which makes sense
>> because the unmapping() will get retried 3 + 7 = 10 times.
> Thanks!  What is the test results for
>
> https://lore.kernel.org/all/20240801081657.1386743-1-dev.jain@arm.com/
>
> ?

That solves the problem completely, makes the test pass. Although
that still may not be a good solution since it solves the problem
for this particular case (reader thread faulting and raising refcount).
As David notes, a concurrent read()/write() should also create this
refcount race problem.


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