Re: [PATCH v2] mm/migrate: put dest folio on deferred split list if source was there.

[Zi Yan]

On 12 Mar 2024, at 12:38, Matthew Wilcox wrote:

> On Tue, Mar 12, 2024 at 11:51:13AM -0400, Zi Yan wrote:
>> On 12 Mar 2024, at 10:19, Matthew Wilcox wrote:
>>
>>> On Tue, Mar 12, 2024 at 10:13:16AM -0400, Zi Yan wrote:
>>>> On 11 Mar 2024, at 23:45, Matthew Wilcox wrote:
>>>>> Much more important: You're doing this with a positive refcount, which
>>>>> breaks the (undocumented) logic in deferred_split_scan() that a folio
>>>>> with a positive refcount will not be removed from the list.
>>>>
>>>> What is the issue here? I thought as long as the split_queue_lock is held,
>>>> it should be OK to manipulate the list.
>>>
>>> I just worked this out yesterday:
>>> https://lore.kernel.org/linux-mm/Ze9EFdFLXQEUVtKl@xxxxxxxxxxxxxxxxxxxx/
>>> (the last chunk, starting with Ryan asking me "what about the first bug
>>> you found")
>>
>> Hmm, like you said a folio with a positive refcount will not be removed
>> from ds_queue->split_queue, it will have no chance going to the separate
>> list in deferred_list_scan() and list_del_init() will not corrupt
>> that list.
>
> You've misread it.  Folios with a _zero_ refcount are not removed from
> the list in deferred_split_scan.  Folios with a positive refcount are
> removed from the per-node or per-cgroup list _at which point there is
> an undocumented assumption_ that they will not be removed from the
> local list because they have a positive refcount.

But that sounds very subtle if not broken. As an outsider of
deferred_split_scan(), only !list_empty(folio->_deferred_list) is checked.
The condition can be true if the folio is on split_queue or
local list of deferred_split_scan() with elevated refcount. In that case,
the folio cannot be removed from the list (either split_queue or local list)
even if split_queue_lock is held, since local list manipulation is not under
split_queue_lock. This makes _deferred_list a one-way train to anyone
except deferred_split_scan(), namely folios can only be added into
_deferred_list until they are freed or split by deferred_split_scan().

Is that intended? If yes, maybe we should document it. If not, using
split_queue_lock to protect local list, or more explicitly folio->_deferred_list
might be better?


>> So it should be safe. Or the issue is that before migration
>> adding a refcount, the folio is removed from ds_queue->split_queue
>> and put on the list in deferred_list_scan(), as a result, any manipulation
>> of folio->_deferred_list could corrupt the list. Basically,
>> !list_empty(&folio->_deferred_list) cannot tell if the folio is on
>> ds_queue->split_queue or another list. I am not sure about why "a positive
>> refcount" is related here.
>>
>> That makes me wonder whether ds_queue->split_queue_lock is also needed
>> for list_for_each_entry_safe() in deferred_split_scan(). Basically,
>> ds_queue->split_queue_lock protects folio->_deferred_list in addition to
>> ds_queue->split_queue.
>>
>>
>>
>>>>> Maximally important: Wer shouldn't be doing any of this!  This folio is
>>>>> on the deferred split list.  We shouldn't be migrating it as a single
>>>>> entity; we should be splitting it now that we're in a context where we
>>>>> can do the right thing and split it.  Documentation/mm/transhuge.rst
>>>>> is clear that we don't split it straight away due to locking context.
>>>>> Splitting it on migration is clearly the right thing to do.
>>>>>
>>>>> If splitting fails, we should just fail the migration; splitting fails
>>>>> due to excess references, and if the source folio has excess references,
>>>>> then migration would fail too.
>>>>
>>>> You are suggesting:
>>>> 1. checking if the folio is on deferred split list or not
>>>> 2. if yes, split the folio
>>>> 3. if split fails, fail the migration as well.
>>>>
>>>> It sounds reasonable to me. The split folios should be migrated since
>>>> the before-split folio wants to be migrated. This split is not because
>>>> no new page cannot be allocated, thus the split folios should go
>>>> into ret_folios list instead of split_folios list.
>>>
>>> Yes, I'm happy for the split folios to be migrated.  Bonus points if you
>>> want to figure out what order to split the folio to ;-)  I don't think
>>> it's critical.
>>
>>
>> --
>> Best Regards,
>> Yan, Zi


--
Best Regards,
Yan, Zi

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