linux-kernel - Re: [PATCH 2/4] ext4: fix incorrect punch max

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Message-ID: <8c1f9230-a475-4fc3-9b2d-5f11f5122bb3@huaweicloud.com>
Date: Wed, 30 Apr 2025 16:44:25 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Jan Kara <jack@...e.cz>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
 wanghaichi0403@...il.com, yi.zhang@...wei.com, libaokun1@...wei.com,
 yukuai3@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH 2/4] ext4: fix incorrect punch max_end

On 2025/4/30 16:18, Jan Kara wrote:
> On Wed 30-04-25 09:12:59, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@...wei.com>
>>
>> For the extents inodes, the maxbytes should be sb->s_maxbytes instead of
>> sbi->s_bitmap_maxbytes. Correct the maxbytes value to correct the
>> behavior of punch hole.
>>
>> Fixes: 2da376228a24 ("ext4: limit length to bitmap_maxbytes - blocksize in punch_hole")
>> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> 
> Thinking about this some more...
> 
>> @@ -4015,6 +4015,12 @@ int ext4_punch_hole(struct file *file, loff_t offset, loff_t length)
>>  	trace_ext4_punch_hole(inode, offset, length, 0);
>>  	WARN_ON_ONCE(!inode_is_locked(inode));
>>  
>> +	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
>> +		max_end = sb->s_maxbytes;
>> +	else
>> +		max_end = EXT4_SB(sb)->s_bitmap_maxbytes;
>> +	max_end -= sb->s_blocksize;
> 
> I think the -= sb->s_blocksize is needed only for indirect-block based
> scheme (due to an implementation quirk in ext4_ind_remove_space()). But
> ext4_ext_remove_space() should be fine with punch hole ending right at
> sb->s_maxbytes. And since I find it somewhat odd that you can create file
> upto s_maxbytes but cannot punch hole to the end, it'd limit that behavior
> as much as possible. Ideally we'd fix ext4_ind_remove_space() but I can't
> be really bothered for the ancient format...
> 

Yes, I share your feelings. Currently, we do not seem to have any
practical issues. To maintain consistent behavior between the two inode
types and to keep the code simple, I retained the -= sb->s_blocksize
operation. Would you suggest that we should at least address the extents
inodes by removing the -=sb->s_blocksize now?

Thanks,
Yi.