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

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Message-ID: <4u2frbxygagij6uxryijqmzgarhotk4cw2w4knm4rpivll5qvg@2d2wd2742v36>
Date: Wed, 30 Apr 2025 12:09:37 +0200
From: Jan Kara <jack@...e.cz>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: Jan Kara <jack@...e.cz>, 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 Wed 30-04-25 16:44:25, Zhang Yi wrote:
> 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?

Yes, what I'm suggesting is that we keep -=sb->s_blocksize specific for the
case !ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR