Re: [PATCH] fs/remap_range: avoid spurious writeback on zero length request

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Nov 28, 2022 at 11:08:13AM -0500, Brian Foster wrote:
> generic_remap_checks() can reduce the effective request length (i.e.,
> after the reflink extend to EOF case is handled) down to zero. If this
> occurs, __generic_remap_file_range_prep() proceeds through dio
> serialization, file mapping flush calls, and may invoke file_modified()
> before returning back to the filesystem caller, all of which immediately
> check for len == 0 and return.
> 
> While this is mostly harmless, it is spurious and not completely
> without side effect. A filemap write call can submit I/O (but not
> wait on it) when the specified end byte precedes the start but
> happens to land on the same aligned page boundary, which can occur
> from __generic_remap_file_range_prep() when len is 0.
> 
> The dedupe path already has a len == 0 check to break out before doing
> range comparisons. Lift this check a bit earlier in the function to
> cover the general case of len == 0 and avoid the unnecessary work.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

Looks correct,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Should there be an(other) "if (!*len) return 0;" after the
generic_remap_check_len call to skip the mtime update if the remap
request gets shortened to avoid remapping an unaligned eofblock into the
middle of the destination file?

--D

> ---
>  fs/remap_range.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/remap_range.c b/fs/remap_range.c
> index 654912d06862..32ea992f9acc 100644
> --- a/fs/remap_range.c
> +++ b/fs/remap_range.c
> @@ -306,6 +306,8 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
>  			remap_flags);
>  	if (ret)
>  		return ret;
> +	if (*len == 0)
> +		return 0;
>  
>  	/* Wait for the completion of any pending IOs on both files */
>  	inode_dio_wait(inode_in);
> @@ -328,9 +330,6 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
>  	if (remap_flags & REMAP_FILE_DEDUP) {
>  		bool		is_same = false;
>  
> -		if (*len == 0)
> -			return 0;
> -
>  		if (!IS_DAX(inode_in))
>  			ret = vfs_dedupe_file_range_compare(file_in, pos_in,
>  					file_out, pos_out, *len, &is_same);
> -- 
> 2.37.3
>