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

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Date: Mon, 17 Jun 2024 11:52:20 +0200
From: David Hildenbrand <david@...hat.com>
To: yangge1116 <yangge1116@....com>, Matthew Wilcox <willy@...radead.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 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 17.06.24 11:50, yangge1116 wrote:
> 
> 
> 在 2024/6/12 下午3:32, David Hildenbrand 写道:
>> On 11.06.24 13:20, yangge1116 wrote:
>>>
>>>
>>> 在 2024/6/9 上午12:03, David Hildenbrand 写道:
>>>> On 08.06.24 17:15, Matthew Wilcox wrote:
>>>>> On Sat, Jun 08, 2024 at 12:38:49PM +0800, yangge1116 wrote:
>>>>>> Can we add a PG_lru_batch flag to determine whether a page is in lru
>>>>>> batch?
>>>>>> If we can, seems this problem will be easier.
>>>>>
>>>>> Page flags are in short supply.  You'd need a really good
>>>>> justification.
>>>>>
>>>>
>>>> A flag would not be able to handle the "part of multiple LRU batches"
>>>> that should currently possible (when to clear the flag?). Well, if we
>>>> have to keep supporting that. If we only to be part in a single LRU
>>>> batch, a new flag could work and we could still allow isolating a folio
>>>> from LRU while in some LRU batch.
>>>
>>> Yes, before adding a folio to LRU batch, check whether the folio has
>>> been added. Add the folio to LRU batch only if the folio has not been
>>> added.
>>>
>>>>
>>>> If we could handle it using the existing flags, that would of course be
>>>> better (wondering if we could store more information in the existing
>>>> flags by using a different encoding for the different states).
>>>
>>> If a folio contains more than one page, the folio will not be added to
>>> LRU batch. Can we use folio_test_large(folio) to filter?
>>>
>>> if (!folio_test_large(folio) && drain_allow) {
>>>      lru_add_drain_all();
>>>      drain_allow = false;
>>> }
>>
>> I think we should do better than this, and not do arbitrary
>> lru_add_drain_all() calls.
>>
> 
> Thanks, I've got another idea.
> 
> If we add GUP_PIN_COUNTING_BIAS to folio's ref count before adding to
> LRU batch, we can use folio_maybe_dma_pinned(folio) to check whether the
> folio is in LRU batch. I wonder if it's feasible?

Why would we want to make folio_maybe_dma_pinned() detection that worse?

-- 
Cheers,

David / dhildenb