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Subject: Re: [PATCH] mm/gup: don't check page lru flag before draining it
To: David Hildenbrand <david@redhat.com>, akpm@linux-foundation.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>
From: yangge1116 <yangge1116@126.com>
Message-ID: <dc7a0b61-8d3f-7205-2f6d-c2b12500947a@126.com>
Date: Wed, 5 Jun 2024 09:18:06 +0800
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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);
     }
}