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

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Message-ID: <20251110174033.GG2988753@mit.edu>
Date: Mon, 10 Nov 2025 12:40:33 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: Pankaj Raghav <kernel@...kajraghav.com>
Cc: libaokun@...weicloud.com, 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 Mon, Nov 10, 2025 at 04:34:47PM +0100, Pankaj Raghav wrote:
> 
> I added something similar for block devices[1]. Probably we might need something
> here as well as a stop gap.
>
> [1] https://lore.kernel.org/all/20250704092134.289491-1-p.raghav@samsung.com/

Yeah, this is the precisely code that I ran into; it's good that we're
not triggering a panic if we try mounting a file system with a large
block size, but when trying to mount file system with a large
blocksize w/o CONFIG_TRANSPARENT_HUGEPAGE, we get:

[   33.211382] XFS (vdc): block size (65536 bytes) not supported; Only block size (4096) or less is supported
mount: /vdc: fsconfig() failed: Function not implemented.
       dmesg(1) may have more information after failed mount system call.

or

[   78.537420] EXT4-fs (vdc): bad block size 65536

Pity the poor user who is trying to use large block sizes, and who
didn't bother to enabl transparent hugepages because they didn't need
it.  Fortunately most distributions tend to enable THP.

> 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 :)

Cool!  So if we're going to change it, perhaps we should have an
explicit CONFIG option, say, CONFIG_FS_LARGE_BLOCKSIZE which enables
bs > ps.  This might allow us to remove smount of code for those
embedded applications who don't need large block sizes, but more
importantly, we can have it automatically enable whatever depedencies
that are needed --- and if it changes later, we can have the kernel
config DTRT automatically.

						- Ted