linux-kernel - Re: [RFC PATCH] mm: don't promote exclusive file folios of dying processes

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Message-ID: <d5cd2055-62ea-4534-b5e2-c6a5bfa9b1c4@redhat.com>
Date: Wed, 16 Apr 2025 11:40:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Johannes Weiner <hannes@...xchg.org>, Matthew Wilcox <willy@...radead.org>,
 Oscar Salvador <osalvador@...e.de>, Ryan Roberts <ryan.roberts@....com>,
 Zi Yan <ziy@...dia.com>
Subject: Re: [RFC PATCH] mm: don't promote exclusive file folios of dying
 processes

On 16.04.25 11:38, Barry Song wrote:
> On Wed, Apr 16, 2025 at 5:32 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 16.04.25 11:24, Barry Song wrote:
>>> On Wed, Apr 16, 2025 at 4:32 PM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> On 12.04.25 10:58, Barry Song wrote:
>>>>> From: Barry Song <v-songbaohua@...o.com>
>>>>>
>>>>> Promoting exclusive file folios of a dying process is unnecessary and
>>>>> harmful. For example, while Firefox is killed and LibreOffice is
>>>>> launched, activating Firefox's young file-backed folios makes it
>>>>> harder to reclaim memory that LibreOffice doesn't use at all.
>>>>
>>>> Do we know when it is reasonable to promote any folios of a dying process?
>>>>
>>>
>>> I don't know. It seems not reasonable at all. if one service crashes due to
>>> SW bug, systemd will restart it immediately. this might be the case promoting
>>> folios might be good. but it is really a bug of the service, not a normal case.
>>>
>>>> Assume you restart Firefox, would it really matter to promote them when
>>>> unmapping? New Firefox would fault-in / touch the ones it really needs
>>>> immediately afterwards?
>>>
>>> Usually users kill firefox to start other applications (users intend
>>> to free memory
>>> for new applications). For Android, an app might be killed because it has been
>>> staying in the background inactively for a while.
>>
>>> On the other hand, even if users restart firefox immediately, their folios are
>>> probably still in LRU to hit.
>>
>> Right, that's what I'm thinking.
>>
>> So I wonder if we could just say "the whole process is going down; even
>> if we had some recency information, that could only affect some other
>> process, where we would have to guess if it really matters".
>>
>> If the data is important, one would assume that another process would
>> soon access it either way, and as you say, likely it will still be on
>> the LRU to hit.
> 
> I'll include this additional information in the v2 version of the patch since
> you think it would be helpful.
> 
> Regarding the exclusive flag - I'm wondering whether we actually need to
> distinguish between exclusive and shared folios in this case. The current
> patch uses the exclusive flag mainly to reduce controversy, but even for
> shared folios: does the recency from a dying process matter? The
> recency information only reflects the dying process's usage pattern, which
> will soon be irrelevant.

Exactly my thoughts. So if we can simplify -- ignore it completely -- 
that would certainly be nice.

-- 
Cheers,

David / dhildenb