Re: [PATCH v3] vfs: fix copy_file_range regression in cross-fs copies

[Christoph Hellwig]

On Wed, Feb 17, 2021 at 05:26:54PM +0000, Luis Henriques wrote:
> A regression has been reported by Nicolas Boichat, found while using the
> copy_file_range syscall to copy a tracefs file.  Before commit
> 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices") the
> kernel would return -EXDEV to userspace when trying to copy a file across
> different filesystems.  After this commit, the syscall doesn't fail anymore
> and instead returns zero (zero bytes copied), as this file's content is
> generated on-the-fly and thus reports a size of zero.
> 
> This patch restores some cross-filesystems copy restrictions that existed
> prior to commit 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
> devices").  It also introduces a flag (COPY_FILE_SPLICE) that can be used
> by filesystems calling directly into the vfs copy_file_range to override
> these restrictions.  Right now, only NFS needs to set this flag.

No need for the flag.  Jyst fall back to splicing in the only caller
that wants it.