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

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Date: Wed, 5 Jun 2024 13:41:50 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 yangge1116 <yangge1116@....com>, akpm@...ux-foundation.org,
 Matthew Wilcox <willy@...radead.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, liuzixing@...on.cn
Subject: Re: [PATCH] mm/gup: don't check page lru flag before draining it

On 05.06.24 13:37, Baolin Wang wrote:
> 
> 
> On 2024/6/5 17:53, David Hildenbrand wrote:
>> 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@....com wrote:
>>>>>> From: yangge <yangge1116@....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)) {
> 
> IMO, this seems violate the semantics of the LRU flag, since it's clear
> that this folio is still in the LRU list.

Good point.

But regarding "violation": we already do clear the flag temporarily in 
there, so it's rather that we make the visible time where it is cleared 
"longer". (yes, it can be much longer :) )

-- 
Cheers,

David / dhildenb