Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Feb 12, 2021 at 2:40 PM Luis Henriques <lhenriques@suse.de> wrote:
>
> Greg KH <gregkh@linuxfoundation.org> writes:
>
> > On Fri, Feb 12, 2021 at 12:05:14PM +0000, Luis Henriques wrote:
> >> Greg KH <gregkh@linuxfoundation.org> writes:
> >>
> >> > On Fri, Feb 12, 2021 at 10:22:16AM +0200, Amir Goldstein wrote:
> >> >> On Fri, Feb 12, 2021 at 9:49 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> >> >> >
> >> >> > On Fri, Feb 12, 2021 at 12:44:00PM +0800, Nicolas Boichat wrote:
> >> >> > > Filesystems such as procfs and sysfs generate their content at
> >> >> > > runtime. This implies the file sizes do not usually match the
> >> >> > > amount of data that can be read from the file, and that seeking
> >> >> > > may not work as intended.
> >> >> > >
> >> >> > > This will be useful to disallow copy_file_range with input files
> >> >> > > from such filesystems.
> >> >> > >
> >> >> > > Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
> >> >> > > ---
> >> >> > > I first thought of adding a new field to struct file_operations,
> >> >> > > but that doesn't quite scale as every single file creation
> >> >> > > operation would need to be modified.
> >> >> >
> >> >> > Even so, you missed a load of filesystems in the kernel with this patch
> >> >> > series, what makes the ones you did mark here different from the
> >> >> > "internal" filesystems that you did not?
> >> >> >
> >> >> > This feels wrong, why is userspace suddenly breaking?  What changed in
> >> >> > the kernel that caused this?  Procfs has been around for a _very_ long
> >> >> > time :)
> >> >>
> >> >> That would be because of (v5.3):
> >> >>
> >> >> 5dae222a5ff0 vfs: allow copy_file_range to copy across devices
> >> >>
> >> >> The intention of this change (series) was to allow server side copy
> >> >> for nfs and cifs via copy_file_range().
> >> >> This is mostly work by Dave Chinner that I picked up following requests
> >> >> from the NFS folks.
> >> >>
> >> >> But the above change also includes this generic change:
> >> >>
> >> >> -       /* this could be relaxed once a method supports cross-fs copies */
> >> >> -       if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
> >> >> -               return -EXDEV;
> >> >> -
> >> >>
> >> >> The change of behavior was documented in the commit message.
> >> >> It was also documented in:
> >> >>
> >> >> 88e75e2c5 copy_file_range.2: Kernel v5.3 updates
> >> >>
> >> >> I think our rationale for the generic change was:
> >> >> "Why not? What could go wrong? (TM)"
> >> >> I am not sure if any workload really gained something from this
> >> >> kernel cross-fs CFR.
> >> >
> >> > Why not put that check back?
> >> >
> >> >> In retrospect, I think it would have been safer to allow cross-fs CFR
> >> >> only to the filesystems that implement ->{copy,remap}_file_range()...
> >> >
> >> > Why not make this change?  That seems easier and should fix this for
> >> > everyone, right?
> >> >
> >> >> Our option now are:
> >> >> - Restore the cross-fs restriction into generic_copy_file_range()
> >> >
> >> > Yes.
> >> >
> >>
> >> Restoring this restriction will actually change the current cephfs CFR
> >> behaviour.  Since that commit we have allowed doing remote copies between
> >> different filesystems within the same ceph cluster.  See commit
> >> 6fd4e6348352 ("ceph: allow object copies across different filesystems in
> >> the same cluster").
> >>
> >> Although I'm not aware of any current users for this scenario, the
> >> performance impact can actually be huge as it's the difference between
> >> asking the OSDs for copying a file and doing a full read+write on the
> >> client side.
> >
> > Regression in performance is ok if it fixes a regression for things that
> > used to work just fine in the past :)
> >
> > First rule, make it work.
>
> Sure, I just wanted to point out that *maybe* there are other options than
> simply reverting that commit :-)
>
> Something like the patch below (completely untested!) should revert to the
> old behaviour in filesystems that don't implement the CFR syscall.
>
> Cheers,
> --
> Luis
>
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 75f764b43418..bf5dccc43cc9 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1406,8 +1406,11 @@ static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
>                                                        file_out, pos_out,
>                                                        len, flags);
>
> -       return generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
> -                                      flags);
> +       if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
> +               return -EXDEV;
> +       else
> +               generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
> +                                       flags);
>  }
>

Which kernel is this patch based on?

At this point, I am with Dave and Darrick on not falling back to
generic_copy_file_range() at all.

We do not have proof of any workload that benefits from it and the
above patch does not protect from a wierd use case of trying to copy a file
from sysfs to sysfs.

I am indecisive about what should be done with generic_copy_file_range()
called as fallback from within filesystems.

I think the wise choice is to not do the fallback in any case, but this is up
to the specific filesystem maintainers to decide.

Thanks,
Amir.