linux-ext4 - Re: [PATCH] ext4: use vmtruncate() instead of ext4_truncate() in ext4

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Date:	Tue, 17 May 2011 22:19:05 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jiaying Zhang <jiayingz@...gle.com>
CC:	tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: use vmtruncate() instead of ext4_truncate() in
 ext4_setattr()

On 5/17/11 5:59 PM, Jiaying Zhang wrote:
> There is a bug in commit c8d46e41 "ext4: Add flag to files with blocks
> intentionally past EOF" that if we fallocate a file with FALLOC_FL_KEEP_SIZE
> flag and then ftruncate the file to a size larger than the file's i_size,
> any allocated but unwritten blocks will be freed but the file size is set
> to the size that ftruncate specifies.
> 
> Here is a simple test to reproduce the problem:
>   1. fallocate a 12k size file with KEEP_SIZE flag
>   2. write the first 4k
>   3. ftruncate the file to 8k
> Then 'ls -l' shows that the i_size of the file becomes 8k but debugfs
> shows the file has only the first written block left.

To be honest I'm not 100% certain what the fiesystem -should- do in this case.

If I go through that same sequence on xfs, I get 4k written / 8k unwritten:

# xfs_bmap -vp testfile
testfile:
 EXT: FILE-OFFSET      BLOCK-RANGE            AG AG-OFFSET              TOTAL FLAGS
   0: [0..7]:          2648750760..2648750767  3 (356066400..356066407)     8 00000
   1: [8..23]:         2648750768..2648750783  3 (356066408..356066423)    16 10000

size 8k:
# ls -l testfile
-rw-r--r-- 1 root root 8192 May 17 22:33 testfile

and diskspace used 12k:
# du -hc testfile
12K	testfile
12K	total

I think this is a different result from ext4, either with or without your patch.

On ext4 I get size 8k, but only the first 4k mapped, as you say.

I don't recall when truncate is supposed to free fallocated blocks, and from what point?

-Eric

> Below is the proposed patch to fix the bug:
> 
> ext4: use vmtruncate() instead of ext4_truncate() in ext4_setattr().
> 
> Change ext4_setattr() to use vmtruncate(inode, attr->ia_size) instead
> of ext4_truncate(inode) when it needs to truncate an inode so that
> if the inode has EXT4_EOFBLOCKS_FL flag set and we are trying to truncate
> to a size larger than the inode's i_size, we will only truncate the blocks
> beyond the specified truncate size instead of all of blocks beyond i_size.
> 
> Signed-off-by: Jiaying Zhang <jiayingz@...gle.com>
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 3424e82..3bfad57 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5347,8 +5347,11 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
>  			}
>  		}
>  		/* ext4_truncate will clear the flag */
> -		if ((ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS)))
> -			ext4_truncate(inode);
> +		if ((ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS))) {
> +			rc = vmtruncate(inode, attr->ia_size);
> +			if (rc)
> +				goto err_out;
> +		}
>  	}
>  
>  	if ((attr->ia_valid & ATTR_SIZE) &&
> --
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