linux-ext4 - Re: LBS support for EXT4

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Message-ID: <p7ba6n2kgrrmcjlsfcjxuo3xrc6fph4wq6qbwxe6fjur54pt55@pvuxuki43v5r>
Date: Wed, 25 Jun 2025 14:25:13 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Baokun Li <libaokun1@...wei.com>
Cc: Jan Kara <jack@...e.cz>, Luis Chamberlain <mcgrof@...nel.org>, 
	Pankaj Raghav <p.raghav@...sung.com>, linux-ext4@...r.kernel.org, Zhang Yi <yi.zhang@...weicloud.com>, 
	Theodore Ts'o <tytso@....edu>
Subject: Re: LBS support for EXT4

On Wed, Jun 25, 2025 at 07:51:56PM +0800, Baokun Li wrote:
> On 2025/6/25 19:13, Pankaj Raghav (Samsung) wrote:
> > Thanks for the reply, Ted.
> > 
> > On Mon, Jun 23, 2025 at 10:17:53AM -0400, Theodore Ts'o wrote:
> > > If you want to review and test the ext4/iomap changes, that would be
> > > great.  Be aware, though, that there are some features of ext4
> > > (example: data journalling, fscrypt, fsverity, etc.) that the current
> > > iomap buffered I/O code may not support today.  The alternatives are
> > > to keep the existing ext4 code paths for those file system features,
> > > or to try to add that functionality into iomap.  There are of course
> > > tradeoffs to both alternatives; one might result in more code that we
> > > have to maintain; the other might require a lot more work.
> > > 
> > > It _might_ be less effort to add LBS support to native ext4 code.  I
> > > think the main thing is to make sure that we always we use a large
> > > folio and not fall back to a sub-blocksize set of pages.  So again,
> > > it's all about tradeoffs and what you consider to be the highest
> > > priority.
> > @Baokun are your LBS patches based on the native ext4 code or on top of
> > Zhang's iomap patches.
> Now that mainline ext4 supports buffer head large folios, we'll first
> focus on LBS support based on buffer heads. The main work involves adapting
> ext4's internal logic (e.g., block allocation, read/write operations,
> defragmentation) and clean up the process related to buffer head.

Makes sense. When we added the LBS support to XFS, the changes in XFS
itself was very minimal. But I can imagine there is a bit more work and
testing involved in adding LBS support to EXT4 with buffer heads.

> This doesn't conflict with iomap buffer write support. The iomap framework
> already supports LBS (as xfs is already using it), so once ext4's internal
> logic is adapted, Zhang Yi's iomap buffer write patches should also support
> LBS upon their merge.

Yes. iomap has all the logic to handle LBS.

Thanks for the clarification. Looking forward to the patches.

-- 
Pankaj Raghav