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

[Ryan Roberts <ryan.roberts@arm.com>]

On 12/03/2024 03:45, Matthew Wilcox wrote:
> On Mon, Mar 11, 2024 at 03:58:48PM -0400, Zi Yan wrote:
>> @@ -1168,6 +1172,17 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>>  		folio_lock(src);
>>  	}
>>  	locked = true;
>> +	if (folio_test_large_rmappable(src) &&

I think you also need to check that the order > 1, now that we support order-1
pagecache folios? _deferred_list only exists if order > 1.

>> +		!list_empty(&src->_deferred_list)) {
>> +		struct deferred_split *ds_queue = get_deferred_split_queue(src);
>> +
>> +		spin_lock(&ds_queue->split_queue_lock);
>> +		ds_queue->split_queue_len--;
>> +		list_del_init(&src->_deferred_list);
>> +		spin_unlock(&ds_queue->split_queue_lock);
>> +		old_page_state |= PAGE_WAS_ON_DEFERRED_LIST;
>> +	}
> 
> I have a few problems with this ...
> 
> Trivial: your whitespace is utterly broken.  You can't use a single tab
> for both indicating control flow change and for line-too-long.
> 
> Slightly more important: You're checking list_empty outside the lock
> (which is fine in order to avoid unnecessarily acquiring the lock),
> but you need to re-check it inside the lock in case of a race.  And you
> didn't mark it as data_race(), so KMSAN will whinge.

I've seen data_race() used around list_empty() without the lock held
inconsistently (see deferred_split_folio()). What are the rules? Given that we
are not doing a memory access here, I don't really understand why it is needed?
list_empty() uses READ_ONCE() which I thought would be sufficient? (I've just
added a similar lockless check in my swap-out series - will add data_race() if
needed, but previously concluded it's not).

> 
> 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.
> 
> 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.

This comment makes me wonder what we do in split_huge_page_to_list_to_order() if
the target order is greater than 1 and the input folio is on the deferred split
list. Looks like we currently just remove it from the deferred list. Is there a
case for putting any output folios that are still partially mapped back on the
deferred list, or splitting them to a lower order such that all output folios
are fully mapped, and all unmapped portions are freed?