linux-ext4 - Re: [RFC PATCH v3 07/26] iomap: don't increase i

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Date: Thu, 29 Feb 2024 17:20:01 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: djwong@...nel.org, Dave Chinner <david@...morbit.com>,
 linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
 tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz,
 ritesh.list@...il.com, willy@...radead.org, zokeefe@...gle.com,
 yi.zhang@...wei.com, chengzhihao1@...wei.com, yukuai3@...wei.com,
 wangkefeng.wang@...wei.com
Subject: Re: [RFC PATCH v3 07/26] iomap: don't increase i_size if it's not a
 write operation

Hello Christoph!

On 2024/2/29 6:13, Christoph Hellwig wrote:
> On Wed, Feb 28, 2024 at 04:53:32PM +0800, Zhang Yi wrote:
>> So, we have to handle above case for xfs. I suppose we could keep
>> increasing i_size if the zeroed folio is entirely outside of i_size,
>> make sure we could write back and allocate blocks for the
>> zeroed & delayed extent, something like below, any suggestions ?
> 
> Sorry for being dumb, but what was the problem solved by not updating
> the size for ext4 again?  (for unshare I can't see any reason to
> ever update the inode size)
> 

The problem I want to slove by not updating the size for ext4 is
truncate. Now ext4 use iomap_zero_range() for the case of zero
partial blocks, and ext4's truncate is different from xfs.

Let's think about a simple case, we have a reg file with size 3K,
then truncate it to 1K. ext4 first set i_size to 1K and then call
ext4_block_truncate_page() to zero out data after 1K(EOF) through
iomap_zero_range(). But now it will update i_size in
iomap_write_end(), so the size of the file will increase to 4K,
this is wrong. xfs first zero out data through iomap_truncate_page()
and then set file size to 1K, so the file size is 3K->4K->1K.
Although the result is correct, but the increasing in
iomap_zero_range() is also not necessary, so so I'm just gonna
delete the i_size updating in iomap_zero_range(). It's not for
unhare.

Thanks,
Yi.