Re: [PATCH v1 02/11] VFS permit cross device vfs_copy_file_range

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On Fri, Oct 19, 2018 at 6:30 PM Olga Kornievskaia
<olga.kornievskaia@xxxxxxxxx> wrote:
>
> From: Olga Kornievskaia <kolga@xxxxxxxxxx>
>
> Allow copy_file_range to copy between different superblocks but only
> of the same file system types. This feature was of interest to CIFS
> as well as NFS.
>
> This feature is needed by NFSv4.2 to perform file copy operation on
> the same server or file copy between different NFSv4.2 servers.
>
> If a file system's fileoperations copy_file_range operation prohibits
> cross-device copies, fall back to do_splice_direct. This would be
> needed for the NFS (destination) server side implementation of the
> file copy and currently for CIFS.
>
> Besides NFS, there is only 1 implementor of the copy_file_range FS
> operation -- CIFS. CIFS assumes incoming file descriptors are both
> CIFS but it will check if they are coming from different servers and
> return error code to fall back to do_splice_direct.
>
> NFS will allow for copies between different NFS servers.
>
> Adding to the vfs.txt documentation to explicitly warn about allowing
> for different superblocks of the same file type to be passed into the
> copy_file_range for the future users of copy_file_range method.
>
> Signed-off-by: Olga Kornievskaia <kolga@xxxxxxxxxx>
> ---
>  Documentation/filesystems/vfs.txt |  4 +++-
>  fs/read_write.c                   | 13 ++++++-------
>  2 files changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
> index a6c6a8a..5e520de 100644
> --- a/Documentation/filesystems/vfs.txt
> +++ b/Documentation/filesystems/vfs.txt
> @@ -958,7 +958,9 @@ otherwise noted.
>
>    fallocate: called by the VFS to preallocate blocks or punch a hole.
>
> -  copy_file_range: called by the copy_file_range(2) system call.
> +  copy_file_range: called by copy_file_range(2) system call. This method
> +                  works on two file descriptors that might reside on
> +                  different superblocks of the same type of file system.
>
>    clone_file_range: called by the ioctl(2) system call for FICLONERANGE and
>         FICLONE commands.
> diff --git a/fs/read_write.c b/fs/read_write.c
> index c60790f..474e740 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1578,10 +1578,6 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
>             (file_out->f_flags & O_APPEND))
>                 return -EBADF;
>
> -       /* this could be relaxed once a method supports cross-fs copies */
> -       if (inode_in->i_sb != inode_out->i_sb)
> -               return -EXDEV;
> -

You need to hoist this limitation to  clone_file_range() syscall, because
you are not allowed to change user facing behavior.
Maybe you can later add a uapi flag for copy_file_range() to explicitly allow
for cross-sb copy?
Maybe you can add the flag now for internal use  - only nfsv4 will pass that
flag to the vfs helper and system call will verify that flags == 0.

Thanks,
Amir.



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