Re: [PATCH] mm/gup: don't check page lru flag before draining it

[David Hildenbrand <david@redhat.com>]

On 06.06.24 03:35, yangge1116 wrote:
> 
> 
> 在 2024/6/5 下午5:53, David Hildenbrand 写道:
>> On 05.06.24 11:41, David Hildenbrand wrote:
>>> On 05.06.24 03:18, yangge1116 wrote:
>>>>
>>>>
>>>> 在 2024/6/4 下午9:47, David Hildenbrand 写道:
>>>>> On 04.06.24 12:48, yangge1116@126.com wrote:
>>>>>> From: yangge <yangge1116@126.com>
>>>>>>
>>>>>> If a page is added in pagevec, its ref count increases one, remove
>>>>>> the page from pagevec decreases one. Page migration requires the
>>>>>> page is not referenced by others except page mapping. Before
>>>>>> migrating a page, we should try to drain the page from pagevec in
>>>>>> case the page is in it, however, folio_test_lru() is not sufficient
>>>>>> to tell whether the page is in pagevec or not, if the page is in
>>>>>> pagevec, the migration will fail.
>>>>>>
>>>>>> Remove the condition and drain lru once to ensure the page is not
>>>>>> referenced by pagevec.
>>>>>
>>>>> What you are saying is that we might have a page on which
>>>>> folio_test_lru() succeeds, that was added to one of the cpu_fbatches,
>>>>> correct?
>>>>
>>>> Yes
>>>>
>>>>>
>>>>> Can you describe under which circumstances that happens?
>>>>>
>>>>
>>>> If we call folio_activate() to move a page from inactive LRU list to
>>>> active LRU list, the page is not only in LRU list, but also in one of
>>>> the cpu_fbatches.
>>>>
>>>> void folio_activate(struct folio *folio)
>>>> {
>>>>         if (folio_test_lru(folio) && !folio_test_active(folio) &&
>>>>             !folio_test_unevictable(folio)) {
>>>>             struct folio_batch *fbatch;
>>>>
>>>>             folio_get(folio);
>>>>             //After this, folio is in LRU list, and its ref count have
>>>> increased one.
>>>>
>>>>             local_lock(&cpu_fbatches.lock);
>>>>             fbatch = this_cpu_ptr(&cpu_fbatches.activate);
>>>>             folio_batch_add_and_move(fbatch, folio, folio_activate_fn);
>>>>             local_unlock(&cpu_fbatches.lock);
>>>>         }
>>>> }
>>>
>>> Interesting, the !SMP variant does the folio_test_clear_lru().
>>>
>>> It would be really helpful if we could reliably identify whether LRU
>>> batching code has a raised reference on a folio.
>>>
>>> We have the same scenario in
>>> * folio_deactivate()
>>> * folio_mark_lazyfree()
>>>
>>> In folio_batch_move_lru() we do the folio_test_clear_lru(folio).
>>>
>>> No expert on that code, I'm wondering if we could move the
>>> folio_test_clear_lru() out, such that we can more reliably identify
>>> whether a folio is on the LRU batch or not.
>>
>> I'm sure there would be something extremely broken with the following
>> (I don't know what I'm doing ;) ), but I wonder if there would be a way
>> to make something like that work (and perform well enough?).
>>
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 67786cb771305..642e471c3ec5a 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -212,10 +212,6 @@ static void folio_batch_move_lru(struct folio_batch
>> *fbatch, move_fn_t move_fn)
>>           for (i = 0; i < folio_batch_count(fbatch); i++) {
>>                   struct folio *folio = fbatch->folios[i];
>>
>> -               /* block memcg migration while the folio moves between
>> lru */
>> -               if (move_fn != lru_add_fn && !folio_test_clear_lru(folio))
>> -                       continue;
>> -
>>                   folio_lruvec_relock_irqsave(folio, &lruvec, &flags);
>>                   move_fn(lruvec, folio);
>>
>> @@ -255,8 +251,9 @@ static void lru_move_tail_fn(struct lruvec *lruvec,
>> struct folio *folio)
>>     */
>>    void folio_rotate_reclaimable(struct folio *folio)
>>    {
>> -       if (!folio_test_locked(folio) && !folio_test_dirty(folio) &&
>> -           !folio_test_unevictable(folio) && folio_test_lru(folio)) {
>> +       if (folio_test_lru(folio) && !folio_test_locked(folio) &&
>> +           !folio_test_dirty(folio) && !folio_test_unevictable(folio) &&
>> +           folio_test_clear_lru(folio)) {
>>                   struct folio_batch *fbatch;
>>                   unsigned long flags;
>>
>> @@ -354,7 +351,7 @@ static void folio_activate_drain(int cpu)
>>    void folio_activate(struct folio *folio)
>>    {
>>           if (folio_test_lru(folio) && !folio_test_active(folio) &&
>> -           !folio_test_unevictable(folio)) {
>> +           !folio_test_unevictable(folio) &&
>> folio_test_clear_lru(folio)) {
>>                   struct folio_batch *fbatch;
>>
>>                   folio_get(folio);
>> @@ -699,6 +696,8 @@ void deactivate_file_folio(struct folio *folio)
>>           /* Deactivating an unevictable folio will not accelerate
>> reclaim */
>>           if (folio_test_unevictable(folio))
>>                   return;
>> +       if (!folio_test_clear_lru(folio))
>> +               return;
>>
>>           folio_get(folio);
>>           local_lock(&cpu_fbatches.lock);
>> @@ -718,7 +717,8 @@ void deactivate_file_folio(struct folio *folio)
>>    void folio_deactivate(struct folio *folio)
>>    {
>>           if (folio_test_lru(folio) && !folio_test_unevictable(folio) &&
>> -           (folio_test_active(folio) || lru_gen_enabled())) {
>> +           (folio_test_active(folio) || lru_gen_enabled()) &&
>> +           folio_test_clear_lru(folio)) {
>>                   struct folio_batch *fbatch;
>>
>>                   folio_get(folio);
>> @@ -740,7 +740,8 @@ void folio_mark_lazyfree(struct folio *folio)
>>    {
>>           if (folio_test_lru(folio) && folio_test_anon(folio) &&
>>               folio_test_swapbacked(folio) &&
>> !folio_test_swapcache(folio) &&
>> -           !folio_test_unevictable(folio)) {
>> +           !folio_test_unevictable(folio) &&
>> +           folio_test_clear_lru(folio)) {
>>                   struct folio_batch *fbatch;
>>
>>                   folio_get(folio);
> 
> With your changes, we will call folio_test_clear_lru(folio) firstly to
> clear the LRU flag, and then call folio_get(folio) to pin the folio,
> seems a little unreasonable. Normally, folio_get(folio) is called
> firstly to pin the page, and then some other functions is called to
> handle the folio.

Right, if that really matters (not sure if it does) we could do

if (folio_test_lru(folio) && ...
	folio_get(folio);
	if (!folio_test_clear_lru(folio)) {
		folio_put(folio)
	} else {
		struct folio_batch *fbatch;
	}
}

-- 
Cheers,

David / dhildenb