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

[Luis Henriques]

Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> > > ---
>> > > 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.

Cheers,
-- 
Luis


>> - Explicitly opt-out of CFR per-fs and/or per-file as Nicolas' patch does
>
> No.  That way lies constant auditing and someone being "vigilant" for
> the next 30+ years.  Which will not happen.
>
> thanks,
>
> greg k-h