linux-kernel - Re: [PATCH v2 00/24] ext4: enable block size larger than page size

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Message-ID: <4bbea425-e896-4047-b30d-6598ff7e9165@pankajraghav.com>
Date: Mon, 10 Nov 2025 16:34:47 +0100
From: Pankaj Raghav <kernel@...kajraghav.com>
To: Theodore Ts'o <tytso@....edu>, libaokun@...weicloud.com
Cc: linux-ext4@...r.kernel.org, adilger.kernel@...ger.ca, jack@...e.cz,
 linux-kernel@...r.kernel.org, mcgrof@...nel.org, ebiggers@...nel.org,
 willy@...radead.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 chengzhihao1@...wei.com, libaokun1@...wei.com
Subject: Re: [PATCH v2 00/24] ext4: enable block size larger than page size

On 11/10/25 05:32, Theodore Ts'o wrote:
> I've started looking at this patch series and playing with it, and one
> thing which is worth noting is that CONFIG_TRANSPARENT_HUGEPAGE needs
> to be enabled, or else sb_set_blocksize() will fail for block size >
> page size.  This isn't specific to ext4, and maybe I'm missing
> something, but apparently this isn't documented.  I had to go digging
> through the source code to figure out what was needed.
> 
> I wonder if we should have some kind of warning in sb_set_blocksize()
> where if there is an attempt to set a blocksize > page size and
> transparent hugepages is not configured, we issue a printk_once()
> giving a hint to the user that the reason that the mount failed was
> because transparent hugepages wasn't enabled at compile time.
> 

I added something similar for block devices[1]. Probably we might need something
here as well as a stop gap.

> It **really** isn't obvious that large block size support and
> transparent hugepages are linked.

Funny that you mention this because I have talk about this topic:
Decoupling Large Folios from Transparent Huge Pages in LPC under MM MC [2].
You are more than welcome to come to the talk :)

But just a small summary: When large folios were introduced, it used
THP infrastructure for splitting the folios (for example when we do a truncate).

I hope we will soon be able to sort it out so that we don't have
to sprinkle CONFIG_THP everywhere.

--
Pankaj

[1] https://lore.kernel.org/all/20250704092134.289491-1-p.raghav@samsung.com/
[2] https://lpc.events/event/19/contributions/2139/>
> 					- Ted
>