Re: [Ocfs2-devel] [GIT PULL] ocfs2 changes for 2.6.32

Joel Becker
2009-09-18T01:45:16Z

On Thu, Sep 17, 2009 at 09:29:14AM -0700, Linus Torvalds wrote:
> Why would anybody want to hide it at all? Why even the libc hiding?
>
> Nobody is going to use this except for special apps. Let them see what
> they can do, in all its glory.

I expect everyone will use this through cp(1), so that cp(1) can
try to get server-side copy on the network filesystms.
Speaking of "all its glory", what we have now is:

int sys_copyfileat(int oldfd, const char *oldname, int newfd,
const char *newname, int flags, int atflags)

> So I'd suggest something like having two system calls: one to start the
> operation, and one to control it. And for a filesystem that does atomic
> copies, the 'start' one obviously would also finish it, so the 'control'
> it would be a no-op, because there would never be any outstanding ones.
>
> See what I'm saying? It wouldn't complicate _your_ life, but it would
> allow for filesystems that can't do it atomically (or even quickly).
>
> So the first one would be something like
>
> int copyfile(const char *src, const char *dest, unsigned long flags);
>
> which would return:
>
> - zero on success
> - negative (with errno) on error
> - positive cookie on "I started it, here's my cookie". For extra bonus
> points, maybe the cookie would actually be a file descriptor (for
> poll/select users), but it would _not_ be a file descriptor to the
> resulting _file_, it would literally be a "cookie" to the actual
> copyfile event.

Actually, if the cookie is a magic file descriptor, you don't
need ctl. You can play tricks like polling for completoin,
read(magic_fd, &remain, sizeof(loff_t)) for status, and close(magic_fd)
for cancel. Might be a bit overloaded, though.

> and then for ocfs2 you'd never return positive cookies. You'd never have
> to worry about it.

I suspect we'll later take advantage of copyfile's other
modes. I did reflink as reflink only for the simple fact of doing one
thing and well, not because I think copyfile isn't good.

> Then the second interface would be something like
>
> int copyfile_ctrl(long cookie, unsigned long cmd);
>
> where you'd just have some way to wait for completion and ask how much has
> been copied. The 'cmd' would be some set of 'cancel', 'status' or
> 'uninterruptible wait' or whatever, and the return value would again be
>
> - negative (with errno) for errors (copy failed) - cookie released
> - zero for 'done' - cookie released
> - positive for 'percent remaining' or whatever - cookie still valid
>
> and this would be another callback into the filesystem code, but you'd
> never have to worry about it, since you'd never see it (just leave it
> NULL).

I was going to ask about how to fit both calls into one inode
operation, but I see you're giving this as an additional inode
operation.
This leaves us with a simliar-to-reflink inode copyfile op and a
control op:

->copyfile(old_dentry, dir_inode, new_dentry, flags)
->copyfile_ctl(int cookie, unsigned int cmd)

I have to change the flags a little, as my original proposal
didn't handle backoff correctly.

#define COPYFILE_WAIT 0x0001 /* Block until complete */
#define COPYFILE_ATOMIC 0x0002 /* Things copied must be
point-in-time and it must
fail or succeed completely. */
#define COPYFILE_ALLOW_COW 0x0004 /* The filesystem may share data
extents between the source
and target in a Copy-on-Write
fashion. If neither
COPYFILE_ALLOW_COW nor
COPYFILE_REQUIRE_COW are
specified, data extents must
NOT be shared. When neither
COW flag is provided, most
filesystems should return
-ENOTSUPP, as userspace can
do read-write looping
itself */
#define COPYFILE_REQUIRE_COW 0x0008 /* Data extents MUST be shared
between the source and target
in a Copy-on-Write fashion */
#define COPYFILE_UNPRIV_ATTRS 0x0010 /* Unprivileged attributes
should be copied from the
source to the target */
#define COPYFILE_PRIV_ATTRS 0x0020 /* Privileged attributes should
be copied from the source to
the target if the caller has
the necessary privileges */
#define COPYFILE_REQUIRE_ATTRS 0x0040 /* Combined with the other
attribute flags, the call
MUST fail if the caller lacks
the necessary privileges to
copy ever attribute
requested */

#define COPYFILE_SNAPSHOT_ASYNC (COPYFILE_REQUIRE_COW |
COPYFILE_UNPRIV_ATTRS |
COPYFILE_PRIV_ATTRS |
COPYFILE_ATOMIC)
#define COPYFILE_SNAPSHOT_STRICT_ASYNC (COPYFILE_SNAPSHOT_ASYNC |
COPYFILE_REQUIRE_ATTRS)
#define COPYFILE_SNAPSHOT (COPYFILE_SNAPSHOT_ASYNC |
COPYFILE_WAIT)
#define COPYFILE_SNAPSHOT_STRICT (COPYFILE_SNAPSHOT_STRICT_ASYNC |
COPYFILE_WAIT)

> I dunno. The above seems like a fairly simple and powerful interface, and
> I _think_ it would be ok for NFS and CIFS. And in fact, if that whole
> "background copy" ends up being used a lot, maybe even a local filesystem
> would implement it just to get easy overlapping IO - even if it would just
> be a trivial common wrapper function that says "start a thread to do a
> trivial manual copy".

NFS and CIFS folks, please speak up.

Joel

--

"There is no more evil thing on earth than race prejudice, none at
all. I write deliberately -- it is the worst single thing in life
now. It justifies and holds together more baseness, cruelty and
abomination than any other sort of error in the world."
- H. G. Wells

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127