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

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Date: Wed, 5 Jun 2024 09:18:06 +0800
From: yangge1116 <yangge1116@....com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH] mm/gup: don't check page lru flag before draining it



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);
     }
}