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

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Date:	Wed, 18 May 2011 10:46:36 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.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 Wed, May 18, 2011 at 6:59 AM, Jiaying Zhang <jiayingz@...gle.com> 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.
>
> 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;
> +               }
Hi there,
It seems that the error handling has problems.  1)We should handle
error as the below code does. or 2) we can add a OR condition to the
if statement below so that it can handle case here.  I prefer the 2nd
solution.  it looks like:
-                /* ext4_truncate will clear the flag */
-               if ((ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS)))
-                       ext4_truncate(inode);

-	if ((attr->ia_valid & ATTR_SIZE) &&
-	    attr->ia_size != i_size_read(inode))
+	if (((attr->ia_valid & ATTR_SIZE) &&
+	    attr->ia_size != i_size_read(inode)) ||
+          ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS))
		rc = vmtruncate(inode, attr->ia_size);

Yongqiang.
>        }
>
>        if ((attr->ia_valid & ATTR_SIZE) &&


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-- 
Best Wishes
Yongqiang Yang
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