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

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Feb 15, 2021 at 8:57 PM Trond Myklebust <trondmy@hammerspace.com> wrote:
>
> On Mon, 2021-02-15 at 19:24 +0200, Amir Goldstein wrote:
> > On Mon, Feb 15, 2021 at 6:53 PM Trond Myklebust <
> > trondmy@hammerspace.com> wrote:
> > >
> > > On Mon, 2021-02-15 at 18:34 +0200, Amir Goldstein wrote:
> > > > On Mon, Feb 15, 2021 at 5:42 PM Luis Henriques <
> > > > lhenriques@suse.de>
> > > > wrote:
> > > > >
> > > > > Nicolas Boichat reported an issue when trying to use the
> > > > > copy_file_range
> > > > > syscall on a tracefs file.  It failed silently because the file
> > > > > content is
> > > > > generated on-the-fly (reporting a size of zero) and
> > > > > copy_file_range
> > > > > needs
> > > > > to know in advance how much data is present.
> > > > >
> > > > > This commit restores the cross-fs restrictions that existed
> > > > > prior
> > > > > to
> > > > > 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
> > > > > devices")
> > > > > and
> > > > > removes generic_copy_file_range() calls from ceph, cifs, fuse,
> > > > > and
> > > > > nfs.
> > > > >
> > > > > Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
> > > > > devices")
> > > > > Link:
> > > > > https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/
> > > > > Cc: Nicolas Boichat <drinkcat@chromium.org>
> > > > > Signed-off-by: Luis Henriques <lhenriques@suse.de>
> > > >
> > > > Code looks ok.
> > > > You may add:
> > > >
> > > > Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> > > >
> > > > I agree with Trond that the first paragraph of the commit message
> > > > could
> > > > be improved.
> > > > The purpose of this change is to fix the change of behavior that
> > > > caused the regression.
> > > >
> > > > Before v5.3, behavior was -EXDEV and userspace could fallback to
> > > > read.
> > > > After v5.3, behavior is zero size copy.
> > > >
> > > > It does not matter so much what makes sense for CFR to do in this
> > > > case (generic cross-fs copy).  What matters is that nobody asked
> > > > for
> > > > this change and that it caused problems.
> > > >
> > >
> > > No. I'm saying that this patch should be NACKed unless there is a
> > > real
> > > explanation for why we give crap about this tracefs corner case and
> > > why
> > > it can't be fixed.
> > >
> > > There are plenty of reasons why copy offload across filesystems
> > > makes
> > > sense, and particularly when you're doing NAS. Clone just doesn't
> > > cut
> > > it when it comes to disaster recovery (whereas backup to a
> > > different
> > > storage unit does). If the client has to do the copy, then you're
> > > effectively doubling the load on the server, and you're adding
> > > potentially unnecessary network traffic (or at the very least you
> > > are
> > > doubling that traffic).
> > >
> >
> > I don't understand the use case you are describing.
> >
> > Which filesystem types are you talking about for source and target
> > of copy_file_range()?
> >
> > To be clear, the original change was done to support NFS/CIFS server-
> > side
> > copy and those should not be affected by this change.
> >
>
> That is incorrect:
>
> ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file
> *dst,
>  u64 dst_pos, u64 count)
> {
>
>  /*
>  * Limit copy to 4MB to prevent indefinitely blocking an nfsd
>  * thread and client rpc slot. The choice of 4MB is somewhat
>  * arbitrary. We might instead base this on r/wsize, or make it
>  * tunable, or use a time instead of a byte limit, or implement
>  * asynchronous copy. In theory a client could also recognize a
>  * limit like this and pipeline multiple COPY requests.
>  */
>  count = min_t(u64, count, 1 << 22);
>  return vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
> }
>
> You are now explicitly changing the behaviour of knfsd when the source
> and destination filesystem differ.
>
> For one thing, you are disallowing the NFSv4.2 copy offload use case of
> copying from a local filesystem to a remote NFS server. However you are
> also disallowing the copy from, say, an XFS formatted partition to an
> ext4 partition.
>

Got it.
This is easy to solve with a flag COPY_FILE_SPLICE (or something)
that is internal to kernel users.

FWIW, you may want to look at the loop in ovl_copy_up_data()
for improvements to nfsd_copy_file_range().

We can move the check out to copy_file_range syscall:

        if (flags != 0)
                return -EINVAL;

Leave the fallback from all filesystems and check for the
COPY_FILE_SPLICE flag inside generic_copy_file_range().

Thanks,
Amir.