linux-kernel - Re: retiring laptop_mode? was Re: [PATCH] mm: vmscan: always allow writeback during memcg reclaim

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Message-ID: <gweo3wdh3agfavhiky5cloweu4m2hvgzk2j2euckbka5x7n47e@ezjmx7eq7ks5>
Date: Tue, 16 Dec 2025 15:23:53 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Christoph Hellwig <hch@...radead.org>, Jens Axboe <axboe@...nel.dk>, 
	Deepanshu Kartikey <kartikey406@...il.com>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: retiring laptop_mode? was Re: [PATCH] mm: vmscan: always allow
 writeback during memcg reclaim

On Tue, Dec 16, 2025 at 01:52:01PM -0500, Johannes Weiner wrote:
[...]
> 
> From 087f10b8046864f71ebc3a3f3316b097932cbded Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@...xchg.org>
> Date: Mon, 15 Dec 2025 12:57:53 -0500
> Subject: [PATCH] mm/block/fs: remove laptop_mode
> 
> Laptop mode was introduced to save battery, by delaying and
> consolidating writes and thereby maximize the time rotating hard
> drives wouldn't have to spin.
> 
> Luckily, rotating hard drives, with their high spin-up times and power
> draw, are a thing of the past for battery-powered devices. Reclaim has
> also since changed to not write single filesystem pages anymore, and
> regular filesystem writeback is lumpy by design.
> 
> The juice doesn't appear worth the squeeze anymore. The footprint of
> the feature is small, but nevertheless it's a complicating factor in
> mm, block, filesystems. Developers don't think about it, and it likely
> hasn't been tested with new reclaim and writeback changes in years.
> 
> Let's sunset it. Keep the sysctl with a deprecation warning around for
> a few more cycles, but remove all functionality behind it.
> 
> Suggested-by: Christoph Hellwig <hch@...radead.org>
> Message-ID: <aT-xv1BNYabnZB_n@...radead.org>

Is there a need for above message ID? Why not put the whole lore link?

> Acked-by: Jens Axboe <axboe@...nel.dk>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

One nit below and other than that you can add:

Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>

> --- a/include/uapi/linux/sysctl.h
> +++ b/include/uapi/linux/sysctl.h
> @@ -183,7 +183,7 @@ enum
>  	VM_LOWMEM_RESERVE_RATIO=20,/* reservation ratio for lower memory zones */
>  	VM_MIN_FREE_KBYTES=21,	/* Minimum free kilobytes to maintain */
>  	VM_MAX_MAP_COUNT=22,	/* int: Maximum number of mmaps/address-space */
> -	VM_LAPTOP_MODE=23,	/* vm laptop mode */
> +

There are 8 earlier enums here with names like VM_UNUSED* along with
the information on what were they. Should we have something similar for
this one? Something like:

	VM_UNUSED10=23, /* was vm laptop mode */