Re: [PATCH] copy_file_range.2: Kernel v5.12 updates

Steve French
2021-02-28T22:26:34Z

On Sun, Feb 28, 2021 at 1:36 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Sun, Feb 28, 2021 at 1:08 AM Steve French <smfrench@gmail.com> wrote:
> >
> > On Fri, Feb 26, 2021 at 11:43 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Sat, Feb 27, 2021 at 12:19 AM Alejandro Colomar (man-pages)
> > > <alx.manpages@gmail.com> wrote:
> > > >
> > > > Hello Amir, Luis,
> > > >
> > > > On 2/24/21 5:10 PM, Amir Goldstein wrote:
> > > > > On Wed, Feb 24, 2021 at 4:22 PM Luis Henriques <lhenriques@suse.de> wrote:
> > > > >>
> > > > >> Update man-page with recent changes to this syscall.
> > > > >>
> > > > >> Signed-off-by: Luis Henriques <lhenriques@suse.de>
> > > > >> ---
> > > > >> Hi!
> > > > >>
> > > > >> Here's a suggestion for fixing the manpage for copy_file_range(). Note that
> > > > >> I've assumed the fix will hit 5.12.
> > > > >>
> > > > >> man2/copy_file_range.2 | 10 +++++++++-
> > > > >> 1 file changed, 9 insertions(+), 1 deletion(-)
> > > > >>
> > > > >> diff --git a/man2/copy_file_range.2 b/man2/copy_file_range.2
> > > > >> index 611a39b8026b..b0fd85e2631e 100644
> > > > >> --- a/man2/copy_file_range.2
> > > > >> +++ b/man2/copy_file_range.2
> > > > >> @@ -169,6 +169,9 @@ Out of memory.
> > > > >> .B ENOSPC
> > > > >> There is not enough space on the target filesystem to complete the copy.
> > > > >> .TP
> > > > >> +.B EOPNOTSUPP
> > > >
> > > > I'll add the kernel version here:
> > > >
> > > > .BR EOPNOTSUPP " (since Linux 5.12)"
> > >
> > > Error could be returned prior to 5.3 and would be probably returned
> > > by future stable kernels 5.3..5.12 too
> > >
> > > >
> > > > >> +The filesystem does not support this operation >> +.TP
> > > > >> .B EOVERFLOW
> > > > >> The requested source or destination range is too large to represent in the
> > > > >> specified data types.
> > > > >> @@ -187,7 +190,7 @@ refers to an active swap file.
> > > > >> .B EXDEV
> > > > >> The files referred to by
> > > > >> .IR fd_in " and " fd_out
> > > > >> -are not on the same mounted filesystem (pre Linux 5.3).
> > > > >> +are not on the same mounted filesystem (pre Linux 5.3 and post Linux 5.12).
> > > >
> > > > I'm not sure that 'mounted' adds any value here. Would you remove the
> > > > word here?
> > >
> > > See rename(2). 'mounted' in this context is explained there.
> > > HOWEVER, it does not fit here.
> > > copy_file_range() IS allowed between two mounts of the same filesystem instance.
> > >
> > > To make things more complicated, it appears that cross mount clone is not
> > > allowed via FICLONE/FICLONERANGE ioctl, so ioctl_ficlonerange(2) man page
> > > also uses the 'mounted filesystem' terminology for EXDEV
> > >
> > > As things stand now, because of the fallback to clone logic,
> > > copy_file_range() provides a way for users to clone across different mounts
> > > of the same filesystem instance, which they cannot do with the FICLONE ioctl.
> > >
> > > Fun :)
> > >
> > > BTW, I don't know if preventing cross mount clone was done intentionally,
> > > but as I wrote in a comment in the code once:
> > >
> > > /*
> > > * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
> > > * the same mount. Practically, they only need to be on the same file
> > > * system.
> > > */
> > >
> > > >
> > > > It reads as if two separate devices with the same filesystem type would
> > > > still give this error.
> > > >
> > > > Per the LWN.net article Amir shared, this is permitted ("When called
> > > > from user space, copy_file_range() will only try to copy a file across
> > > > filesystems if the two are of the same type").
> > > >
> > > > This behavior was slightly different before 5.3 AFAICR (was it?) ("until
> > > > then, copy_file_range() refused to copy between files that were not
> > > > located on the same filesystem."). If that's the case, I'd specify the
> > > > difference, or more probably split the error into two, one before 5.3,
> > > > and one since 5.12.
> > > >
> > >
> > > True.
> > >
> > > > >
> > > > > I think you need to drop the (Linux range) altogether.
> > > >
> > > > I'll keep the range. Users of 5.3..5.11 might be surprised if the
> > > > filesystems are different and they don't get an error, I think.
> > > >
> > > > I reworded it to follow other pages conventions:
> > > >
> > > > .BR EXDEV " (before Linux 5.3; or since Linux 5.12)"
> > > >
> > > > which renders as:
> > > >
> > > > EXDEV (before Linux 5.3; or since Linux 5.12)
> > > > The files referred to by fd_in and fd_out are not on
> > > > the same mounted filesystem.
> > > >
> > >
> > > drop 'mounted'
> > >
> > > >
> > > > > What's missing here is the NFS cross server copy use case.
> > > > > Maybe:
> >
> > At least for the SMB3 kernel server (ksmbd "cifsd") looks like they use splice.
> > And for the user space CIFS/SMB3 server (like Samba) they have a configurable
> > plug in library interface ("Samba VFS modules") that would allow you
> > to implement
> > cross filesystem copy optimally for your version of Linux and plug
> > this into Samba
> > with little work on your part.
> >
> > > >
> > > > Again, this wasn't true before 5.3, right?
> > > >
> > >
> > > Right.
> > > Actually, v5.3 provides the vfs capabilities for filesystems to support
> > > cross fs copy. I am not sure if NFS already implements cross fs copy in
> > > v5.3 and not sure about cifs. Need to get input from nfs/cis developers
> > > or dig in the release notes for server-side copy.
> >
> > The SMB3 protocol has multiple ways to do "server side copy" (copy
> > offload to the server), some of which would apply to your example.
> > The case of "reflink" in many cases would be most efficient, and is supported
> > by the Linux client (see MS-SMB2 protocol specification section 3.3.5.15.18) but
> > is supported by fewer server file systems, so probably more important
> > to focus on
> > the other mechanisms which are server side copy rather than clone. The most
> > popular way, supported by most servers, is "CopyChunk" - 100s of
> > millions of systems
> > support this (if not more) - see MS-SMB2 protocol specification
> > section 2.2.31.1 and
> > 3.3.5.15.16 - there are various cases where two different SMB3 mounts
> > on the same
> > client could handle cross mount server side copy.
> >
> > There are other mechanisms supported by fewer servers SMB3 ODX/T10 style copy
> > offload (Windows and some others see e.g. Gordon at Nexenta's presentation
> > https://www.slideshare.net/gordonross/smb3-offload-data-transfer-odx)
> > but still popular for virtualization workloads. For this it could be
> > even more common
> > for those to be different mounts on the client. The Linux client does
> > not support
> > the SMB3 ODX/T10 offload yet but it would be good to add support for it.
> > There is a nice description of its additional benefits at
> > https://docs.microsoft.com/en-us/windows-hardware/drivers/storage/offloaded-data-transfer
> >
> > But - yes SMB3 on Linux can have cross mount file copy today, which is
> > far more efficient
>
> Can have? or does have?
> IIUC, server-side copy ability exists for "same cifs fs" for a long time and
> since v5.3, it is available for "same cifs connection", which is not exactly
> the same as "same cifs fs" but also not really different for most people.
> Can you elaborate about that?
> Just assume the server can do anything. What can the Linux client do
> since v5.3 or later?

Inside the SMB3 client (cifs.ko) we check that the file handles provided
are for the same authenticated user to the same server, so
e.g. you could mount //server/share on /mnt1 and //server/anothershare on /mnt2
and do a copy_file_range from /mnt1/file1 to /mnt2/file2 even though these are
different mounts. The cifs client should allow additional cases of cross mount
copy, but at least this helps for various common scenarios and is very widely
supported on most servers as well.

--
Thanks,

Steve