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

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Message-ID: <a39c8602-3c9c-48fd-9bdb-2089ccccd6bc@redhat.com>
Date: Wed, 12 Jun 2024 09:32:01 +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 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.

-- 
Cheers,

David / dhildenb