Re: [PATCH v2] vfs: prevent copy_file_range to copy across devices

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Wed, Feb 17, 2021 at 05:50:35PM -0700, Andreas Dilger wrote:
> On Feb 17, 2021, at 1:08 AM, Amir Goldstein <amir73il@gmail.com> wrote:
> > 
> > You are missing my point.
> > Never mind which server. The server does not *need* to rely on
> > vfs_copy_file_range() to copy files from XFS to ext4.
> > The server is very capable of implementing the fallback generic copy
> > in case source/target fs do not support native {copy,remap}_file_range().
> > 
> > w.r.t semantics of copy_file_range() syscall vs. the fallback to userespace
> > 'cp' tool (check source file size before copy or not), please note that the
> > semantics of CIFS_IOC_COPYCHUNK_FILE are that of the former:
> > 
> >        rc = cifs_file_copychunk_range(xid, src_file.file, 0, dst_file, 0,
> >                                        src_inode->i_size, 0);
> > 
> > It will copy zero bytes if advertised source file size if zero.
> > 
> > NFS server side copy semantics are currently de-facto the same
> > because both the client and the server will have to pass through this
> > line in vfs_copy_file_range():
> > 
> >        if (len == 0)
> >                return 0;
> > 
> > IMO, and this opinion was voiced by several other filesystem developers,
> > the shortend copy semantics are the correct semantics for copy_file_range()
> > syscall as well as for vfs_copy_file_range() for internal kernel users.
> > 
> > I guess what this means is that if the 'cp' tool ever tries an opportunistic
> > copy_file_range() syscall (e.g. --cfr=auto), it may result in zero size copy.
> 
> Having a syscall that does the "wrong thing" when called on two files
> doesn't make sense.  Expecting userspace to check whether source/target
> files supports CFR is also not practical.  This is trivial for the
> kernel to determine and return -EOPNOTSUPP to the caller if the source
> file (procfs/sysfs/etc) does not work with CFR properly.

How does the kernel "know" that a specific file in a specific filesystem
will not work with CFR "properly"?  That goes back to the original patch
which tried to label each and every filesystem type with a
"supported/not supported" type of flag, which was going to be a mess,
especially as it seems that this might be a file-specific thing, not a
filesystem-specific thing.

The goal of the patch _should_ be that the kernel figure it out itself,
but so far no one seems to be able to explain how that can be done :(

So, any hints?

thanks,

greg k-h