Re: [PATCH hotfix] mm: fix crashes from deferred split racing folio migration

[Zi Yan <ziy@nvidia.com>]

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On 3 Jul 2024, at 12:21, David Hildenbrand wrote:

> On 03.07.24 16:30, Zi Yan wrote:
>> On 2 Jul 2024, at 3:40, Hugh Dickins wrote:
>>
>>> Even on 6.10-rc6, I've been seeing elusive "Bad page state"s (often on
>>> flags when freeing, yet the flags shown are not bad: PG_locked had been
>>> set and cleared??), and VM_BUG_ON_PAGE(page_ref_count(page) == 0)s from
>>> deferred_split_scan()'s folio_put(), and a variety of other BUG and WARN
>>> symptoms implying double free by deferred split and large folio migration.
>>>
>>> 6.7 commit 9bcef5973e31 ("mm: memcg: fix split queue list crash when large
>>> folio migration") was right to fix the memcg-dependent locking broken in
>>> 85ce2c517ade ("memcontrol: only transfer the memcg data for migration"),
>>> but missed a subtlety of deferred_split_scan(): it moves folios to its own
>>> local list to work on them without split_queue_lock, during which time
>>> folio->_deferred_list is not empty, but even the "right" lock does nothing
>>> to secure the folio and the list it is on.
>>>
>>> Fortunately, deferred_split_scan() is careful to use folio_try_get(): so
>>> folio_migrate_mapping() can avoid the race by folio_undo_large_rmappable()
>>> while the old folio's reference count is temporarily frozen to 0 - adding
>>> such a freeze in the !mapping case too (originally, folio lock and
>>> unmapping and no swap cache left an anon folio unreachable, so no freezing
>>> was needed there: but the deferred split queue offers a way to reach it).
>>>
>>> Fixes: 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration")
>>> Signed-off-by: Hugh Dickins <hughd@google.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>> This patch against 6.10-rc6: Kefeng has commits in the mm-tree which
>>> which will need adjustment to go over this, but we can both check the
>>> result.  I have wondered whether just reverting 85ce2c517ade and its
>>> subsequent fixups would be better: but that would be a bigger job,
>>> and probably not the right choice.
>>>
>>>   mm/memcontrol.c | 11 -----------
>>>   mm/migrate.c    | 13 +++++++++++++
>>>   2 files changed, 13 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index 71fe2a95b8bd..8f2f1bb18c9c 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -7823,17 +7823,6 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
>>>
>>>   	/* Transfer the charge and the css ref */
>>>   	commit_charge(new, memcg);
>>> -	/*
>>> -	 * If the old folio is a large folio and is in the split queue, it needs
>>> -	 * to be removed from the split queue now, in case getting an incorrect
>>> -	 * split queue in destroy_large_folio() after the memcg of the old folio
>>> -	 * is cleared.
>>> -	 *
>>> -	 * In addition, the old folio is about to be freed after migration, so
>>> -	 * removing from the split queue a bit earlier seems reasonable.
>>> -	 */
>>> -	if (folio_test_large(old) && folio_test_large_rmappable(old))
>>> -		folio_undo_large_rmappable(old);
>>>   	old->memcg_data = 0;
>>>   }
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 20cb9f5f7446..a8c6f466e33a 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -415,6 +415,15 @@ int folio_migrate_mapping(struct address_space *mapping,
>>>   		if (folio_ref_count(folio) != expected_count)
>>>   			return -EAGAIN;
>>>
>>> +		/* Take off deferred split queue while frozen and memcg set */
>>> +		if (folio_test_large(folio) &&
>>> +		    folio_test_large_rmappable(folio)) {
>>> +			if (!folio_ref_freeze(folio, expected_count))
>>> +				return -EAGAIN;
>>> +			folio_undo_large_rmappable(folio);
>>> +			folio_ref_unfreeze(folio, expected_count);
>>> +		}
>>> +
>>
>> I wonder if the patch below would make the code look better by using
>> the same freeze/unfreeze pattern like file-backed path. After
>> reading the emails between you and Baolin and checking the code,
>> I think the patch looks good to me. Feel free to add
>> Reviewed-by: Zi Yan <ziy@nvidia.com>
>>
>> BTW, this subtlety is very error prone, as Matthew, Ryan, and I all
>> encountered errors because of this[1][2]. Matthew has a good summary
>> of the subtlety:
>>
>> "the (undocumented) logic in deferred_split_scan() that a folio
>> with a positive refcount will not be removed from the list."
>>
>> [1] https://lore.kernel.org/linux-mm/Ze9EFdFLXQEUVtKl@casper.infradead.org/
>> [2] https://lore.kernel.org/linux-mm/Ze_P6xagdTbcu1Kz@casper.infradead.org/
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index a8c6f466e33a..afcc0653dcb7 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -412,17 +412,15 @@ int folio_migrate_mapping(struct address_space *mapping,
>>
>>          if (!mapping) {
>>                  /* Anonymous page without mapping */
>> -               if (folio_ref_count(folio) != expected_count)
>> +               if (!folio_ref_freeze(folio, expected_count))
>>                          return -EAGAIN;
>>
>>                  /* Take off deferred split queue while frozen and memcg set */
>>                  if (folio_test_large(folio) &&
>> -                   folio_test_large_rmappable(folio)) {
>> -                       if (!folio_ref_freeze(folio, expected_count))
>> -                               return -EAGAIN;
>> +                   folio_test_large_rmappable(folio))
>>                          folio_undo_large_rmappable(folio);
>> -                       folio_ref_unfreeze(folio, expected_count);
>> -               }
>> +
>> +               folio_ref_unfreeze(folio, expected_count);
>>
>
> The downside is freezing order-0, where we don't need to freeze, right?

Right. I missed that part. Forget about my change above. Thanks.


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Best Regards,
Yan, Zi

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