Re: [PATCH] vfs: Export fallocate facility to kernel modules

[Tyler Hicks <tyhicks@canonical.com>]

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On 2011-10-31 12:14:05, Thieu Le wrote:
> Export fallocate facility so layered file systems such as ecryptfs can
> take advantage of this feature.  One example of the need for fallocate
> is by ecryptfs.  ecryptfs has recently switched to a writeback cache
> model so its dirty pages are not written to the lower file system
> immediately.  In order to ensure that space is available when the page
> is later written, ecryptfs can efficiently preallocate that space using
> vfs_fallocate().

Hi Thieu - Thanks for the patch. Something like this is definitely
needed for eCryptfs.

> 
> vfs_fallocate() does not perform all of the checkings of do_fallocate()
> because those checks are done higher in the stack.

After taking a closer look at the checks in do_fallocate(), I don't know
that this statement is correct. The sanity checks around offset, len,
and mode aren't guaranteed to be done higher up in the stack. The
existing VFS helper functions also seem to do these types of checks. I
think the rest of the checks are redundant.

Tyler

> Checking of offset and
> len are done by rw_verify_area() and generic_write_checks().  These
> functions are called by VFS before ecryptfs invokes vfs_fallocate() in its
> ecryptfs_write_end() function.  And since ecryptfs directly opens the lower
> file, it ensures that the file is opened with the proper flags and mode.
> 
> Signed-off-by: Thieu Le <thieule@chromium.org>
> ---
>  fs/open.c          |   14 ++++++++++----
>  include/linux/fs.h |    2 ++
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/open.c b/fs/open.c
> index f711921..8a4ba20 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -214,6 +214,15 @@ SYSCALL_ALIAS(sys_ftruncate64, SyS_ftruncate64);
>  #endif /* BITS_PER_LONG == 32 */
>  
>  
> +int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> +{
> +	if (!file->f_op->fallocate)
> +		return -EOPNOTSUPP;
> +
> +	return file->f_op->fallocate(file, mode, offset, len);
> +}
> +EXPORT_SYMBOL(vfs_fallocate);
> +
>  int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>  {
>  	struct inode *inode = file->f_path.dentry->d_inode;
> @@ -263,10 +272,7 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>  	if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
>  		return -EFBIG;
>  
> -	if (!file->f_op->fallocate)
> -		return -EOPNOTSUPP;
> -
> -	return file->f_op->fallocate(file, mode, offset, len);
> +	return vfs_fallocate(file, mode, offset, len);
>  }
>  
>  SYSCALL_DEFINE(fallocate)(int fd, int mode, loff_t offset, loff_t len)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 277f497..c0e1225 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1504,6 +1504,8 @@ extern int vfs_link(struct dentry *, struct inode *, struct dentry *);
>  extern int vfs_rmdir(struct inode *, struct dentry *);
>  extern int vfs_unlink(struct inode *, struct dentry *);
>  extern int vfs_rename(struct inode *, struct dentry *, struct inode *, struct dentry *);
> +extern int vfs_fallocate(struct file *file, int mode, loff_t offset,
> +			 loff_t len);
>  
>  /*
>   * VFS dentry helper functions.
> -- 
> 1.7.3.1
> 
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