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

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Message-ID: <617f9e36-9334-4630-a6b9-473f2dd570d4@redhat.com>
Date: Sat, 8 Jun 2024 18:03:55 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>, yangge1116 <yangge1116@....com>
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 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.

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).

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 :)

-- 
Cheers,

David / dhildenb