linux-ext4 - Re: LBS support for EXT4

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Message-ID: <23915ba3-5c00-4628-a22a-3fdcd4ad0b62@huaweicloud.com>
Date: Sat, 21 Jun 2025 10:54:39 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Jan Kara <jack@...e.cz>, Pankaj Raghav <kernel@...kajraghav.com>
Cc: tytso@....edu, Luis Chamberlain <mcgrof@...nel.org>,
 Pankaj Raghav <p.raghav@...sung.com>, linux-ext4@...r.kernel.org
Subject: Re: LBS support for EXT4

Hi Pankaj and Jan,

On 2025/6/20 23:55, Jan Kara wrote:
> [added ext4 list and Zhang to CC]
> 
> Hi,
> 
> On Thu 19-06-25 15:05:14, Pankaj Raghav wrote:
>> Hello Jan and Ted,
>>
>> As you might know, I added LBS support to XFS sometime back. And after
>> that Luis added LBS support to block devices/buffer head path.
>>
>> And now that EXT4 supports large folios, it should be possible to add LBS
>> support to EXT4.
>>
>> I started digging in to the code, and it looks like it might require some
>> rework throughout EXT4 and lot of testing.
>>
>> My question is:
>>
>> I have seen patches from Zhang Yi to add iomap buffered IO support to EXT4.
>> If that happens, then adding LBS support should become trivial.
>>
>> Do you think it might happen soon or it is going to take more time?
>> Seeing the patches it is hard for me to say what the status is as the
>> last patches posted for them was last year.
> 
> Well, time is always relative so it's difficult to tell what do you mean by
> "soon" :). We are definitely interested in converting ext4 to iomap.
> Currently we are fixing up some remaining issues caused by conversion to
> support large order folios but after that iomap conversion would be a next
> logical step. Zhang Yi had patches for that, I'm not sure how much from
> them is left to apply after the large order folios have landed.

We have been working on the conversion of ext4 to iomap. The conversion
process for iomap has proven to be more complex than anticipated due to
numerous prerequisite issues and necessary refactors that must be
integrated. Currently, all prerequisite patches(perhaps) have been merged
into the latest 6.16-rc1 release, so I am rebasing my iomap series recently.
However, I have encountered some blocking issues during the rebase, and
I am trying to resolve them. I hope to release it as soon as possible.

> 
>> The reason I am asking is, should I take up the challenge to add LBS
>> support with buffer heads in EXT4, or should I wait until iomap patches
>> are merged.
> 
> I think better spent time would be to help with the iomap conversion. I
> don't think there will be that much coding left (perhaps some more exotic
> features need attention) but there's definitely testing needed and review
> is always welcome and most needed...
> 

My colleagues, Baokun and Zhihao, have been working on LBS support for a
couple of months, based on my two large folio series (buffer_head and
iomap convesion) and the bdev large folio series form Luis. They will
release the first version after testing as soon as possible.

Best regards,
Yi.