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

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Date: Tue, 11 Jun 2024 19:20:26 +0800
From: yangge1116 <yangge1116@....com>
To: David Hildenbrand <david@...hat.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



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;
}

> 
> The temporary clearing of the LRU flag we do right now tells me that 
> it's already not 100% reliable, so the question is how much more 
> unreliable we can make it before it would hurt :)
>