Received-SPF: pass (google.com: domain of linux-kernel+bounces-203755-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Message-ID: <19590664-5190-4d30-ba0d-ec9d0ea373d3@redhat.com>
Date: Thu, 6 Jun 2024 09:39:14 +0200
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH] mm/gup: don't check page lru flag before draining it
To: yangge1116 <yangge1116@126.com>, akpm@linux-foundation.org,
 Matthew Wilcox <willy@infradead.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
 baolin.wang@linux.alibaba.com, liuzixing@hygon.cn
References: <1717498121-20926-1-git-send-email-yangge1116@126.com>
 <0d7a4405-9a2e-4bd1-ba89-a31486155233@redhat.com>
 <dc7a0b61-8d3f-7205-2f6d-c2b12500947a@126.com>
 <776de760-e817-43b2-bd00-8ce96f4e37a8@redhat.com>
 <7063920f-963a-4b3e-a3f3-c5cc227bc877@redhat.com>
 <a30ee643-8690-16f1-c315-c252d2c16e4a@126.com>
From: David Hildenbrand <david@redhat.com>
Content-Language: en-US
Autocrypt: addr=david@redhat.com; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <a30ee643-8690-16f1-c315-c252d2c16e4a@126.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

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