Re: fallocate mode flag for "unshare blocks"?

On Thu, Mar 31, 2016 at 02:08:21PM -0400, J. Bruce Fields wrote:

On Thu, Mar 31, 2016 at 10:18:50PM +1100, Dave Chinner wrote:

quoted

On Thu, Mar 31, 2016 at 12:54:40AM -0700, Christoph Hellwig wrote:

quoted

On Thu, Mar 31, 2016 at 12:18:13PM +1100, Dave Chinner wrote:

quoted

On Wed, Mar 30, 2016 at 11:27:55AM -0700, Darrick J. Wong wrote:

quoted

Or is it ok that fallocate could block, potentially for a long time as
we stream cows through the page cache (or however unshare works
internally)?  Those same programs might not be expecting fallocate to
take a long time.

Yes, it's perfectly fine for fallocate to block for long periods of
time. See what gfs2 does during preallocation of blocks - it ends up
calling sb_issue_zerout() because it doesn't have unwritten
extents, and hence can block for long periods of time....

gfs2 fallocate is an implementation that will cause all but the most
trivial users real pain.  Even the initial XFS implementation just
marking the transactions synchronous made it unusable for all kinds
of applications, and this is much worse.  E.g. a NFS ALLOCATE operation
to gfs2 will probab;ly hand your connection for extended periods of
time.

If we need to support something like what gfs2 does we should have a
separate flag for it.

Using fallocate() for preallocation was always intended to
be a faster, more efficient method allocating zeroed space
than having userspace write blocks of data. Faster, more efficient
does not mean instantaneous, and gfs2 using sb_issue_zerout() means
that if the hardware has zeroing offloads (deterministic trim, write
same, etc) it will use them, and that will be much faster than
writing zeros from userspace.

IMO, what gfs2 is definitely within the intended usage of
fallocate() for accelerating the preallocation of blocks.

Yes, it may not be optimal for things like NFS servers which haven't
considered that a fallocate based offload operation might take some
time to execute, but that's not a problem with fallocate. i.e.
that's a problem with the nfs server ALLOCATE implementation not
being prepared to return NFSERR_JUKEBOX to prevent client side hangs
and timeouts while the operation is run....

That's an interesting idea, but I don't think it's really legal.  I take
JUKEBOX to mean "sorry, I'm failing this operation for now, try again
later and it might succeed", not "OK, I'm working on it, try again and
you may find out I've done it".

So if the client gets a JUKEBOX error but the server goes ahead and does
the operation anyway, that'd be unexpected.

I suppose it's comparable to the case where a slow fallocate is
interrupted--would it be legal to return EINTR in that case and leave
the application to sort out whether some part of the allocation had
already happened?

<shrug> The unshare component to XFS fallocate does this if something
sends a fatal signal to the process.  There's a difference between
shooting down a process in the middle of fallocate and fallocate
returning EINTR out of the blue, though...

...the manpage for fallocate says that "EINTR == a signal was caught".

Would it be legal to continue the fallocate under the covers even
after returning EINTR?

It doesn't do that, however.

--D

But anyway my first inclination is to say that the NFS FALLOCATE
protocol just wasn't designed to handle long-running fallocates, and if
we really need that then we need to give it a way to either report
partial results or to report results asynchronously.

--b.