Re: [PATCH v4 08/10] NFSD handle OFFLOAD_CANCEL op

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Oct 11, 2017 at 11:02:56AM -0400, Olga Kornievskaia wrote:
> On Wed, Oct 11, 2017 at 10:07 AM, J. Bruce Fields <bfields@fieldses.org> wrote:
> > On Tue, Oct 10, 2017 at 05:14:29PM -0400, Olga Kornievskaia wrote:
> >> On Mon, Oct 9, 2017 at 11:58 AM, J. Bruce Fields <bfields@redhat.com> wrote:
> >> > On Mon, Oct 09, 2017 at 10:53:13AM -0400, Olga Kornievskaia wrote:
> >> >> On Thu, Sep 28, 2017 at 2:38 PM, J. Bruce Fields <bfields@redhat.com> wrote:
> >> >> > On Thu, Sep 28, 2017 at 01:29:43PM -0400, Olga Kornievskaia wrote:
> >> >> >> Upon receiving OFFLOAD_CANCEL search the list of copy stateids,
> >> >> >> if found mark it cancelled. If copy has more interations to
> >> >> >> call vfs_copy_file_range, it'll stop it. Server won't be sending
> >> >> >> CB_OFFLOAD to the client since it received a cancel.
> >> >> >>
> >> >> >> Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
> >> >> >> ---
> >> >> >>  fs/nfsd/nfs4proc.c  | 26 ++++++++++++++++++++++++--
> >> >> >>  fs/nfsd/nfs4state.c | 16 ++++++++++++++++
> >> >> >>  fs/nfsd/state.h     |  4 ++++
> >> >> >>  3 files changed, 44 insertions(+), 2 deletions(-)
> >> >> >>
> >> >> >> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> >> >> >> index 3cddebb..f4f3d93 100644
> >> >> >> --- a/fs/nfsd/nfs4proc.c
> >> >> >> +++ b/fs/nfsd/nfs4proc.c
> >> >> >> @@ -1139,6 +1139,7 @@ static int _nfsd_copy_file_range(struct nfsd4_copy *copy)
> >> >> >>       size_t bytes_to_copy;
> >> >> >>       u64 src_pos = copy->cp_src_pos;
> >> >> >>       u64 dst_pos = copy->cp_dst_pos;
> >> >> >> +     bool cancelled = false;
> >> >> >>
> >> >> >>       do {
> >> >> >>               bytes_to_copy = min_t(u64, bytes_total, MAX_RW_COUNT);
> >> >> >> @@ -1150,7 +1151,12 @@ static int _nfsd_copy_file_range(struct nfsd4_copy *copy)
> >> >> >>               copy->cp_res.wr_bytes_written += bytes_copied;
> >> >> >>               src_pos += bytes_copied;
> >> >> >>               dst_pos += bytes_copied;
> >> >> >> -     } while (bytes_total > 0 && !copy->cp_synchronous);
> >> >> >> +             if (!copy->cp_synchronous) {
> >> >> >> +                     spin_lock(&copy->cps->cp_lock);
> >> >> >> +                     cancelled = copy->cps->cp_cancelled;
> >> >> >> +                     spin_unlock(&copy->cps->cp_lock);
> >> >> >> +             }
> >> >> >> +     } while (bytes_total > 0 && !copy->cp_synchronous && !cancelled);
> >> >> >>       return bytes_copied;
> >> >> >
> >> >> > I'd rather we sent a signal, and then we won't need this
> >> >> > logic--vfs_copy_range() will just return EINTR or something.
> >> >>
> >> >> Hi Bruce,
> >> >>
> >> >> Now that I've implemented using the kthread instead of the workqueue,
> >> >> I don't see that it can provide any better  guarantee than the work
> >> >> queue. vfs_copy_range() is not interrupted in the middle and returning
> >> >> the EINTR. The function that runs the kthread, it has to at some point
> >> >> call signalled()/kthread_should_stop() function to see if it was
> >> >> signaled and use it to 'stop working instead of continuing on'.
> >> >>
> >> >> If I were to remove the loop and check (if signaled() ||
> >> >> kthread_should_stop()) before and after calling the
> >> >> vfs_copy_file_range(), the copy will either not start if the
> >> >> OFFLOAD_CANCEL was received before copy started or the whole copy
> >> >> would happen.
> >> >>
> >> >> Even with the loop, I'd be checking after every call for
> >> >> vfs_copy_file_range() just like it was in the current version with the
> >> >> workqueue.
> >> >>
> >> >> Please advise if you still want the kthread-based implementation or
> >> >> keep the workqueue.
> >> >
> >> > That's interesting.
> >> >
> >> > To me that sounds like a bug somewhere under vfs_copy_file_range().
> >> > splice_direct_to_actor() can do long-running copies, so it should be
> >> > interruptible, shouldn't it?
> >>
> >> So I found it. Yes do_splice_direct() will react to somebody sending a
> >> ctrl-c and will stop. It calls signal_pendning(). However, in our
> >> case, I'm calling kthread_stop() and that sets a different flag and
> >> one needs to also check for kthread_should_stop() as a stopping
> >> condition. splice.c lacks that.
> >>
> >> I hope they can agree that it's a bug. I don't have any luck with VFS...
> >
> > Argh.  No, it's probably not their bug, I guess kthreads just ignore
> > signals.  OK, I can't immediately see what the right thing to do is
> > here....
> >
> > I do think we need to do something as we want to be able to interrupt
> > and clean up copy threads when we can.
> 
> A bug is not the right word. It would be asking them to accommodate
> stopping to include kthread_stop condition. Why do you say kthreads
> ignore signals? You can say that kthread_stop doesn't send a signal.

I think both are true.

I doubt it's reasonable to add kthread_should_stop everywhere that
there are currently checks for signals.

> Also another note, I still can't remove the loop around the call to
> the vfs_copy_file_range() because it's not guaranteed to copy all the
> bytes that the call asks for. The implementation of
> vfs_copy_file_range will do_splice_direct only MAX_RW_COUNT at a time.
> So the upper layer needs to loop to make sure it copies all the bytes.

MAX_RW_COUNT is about 4 gigs.  I'm not sure if it's really a problem to
copy only 4 gigs at a time?  But, yes, maybe the loop is still worth it.

> If VFS will decide to reject the request to add kthread_should_stop to
> their conditions, then the loop could be a way to stop every 4MB.
> Copying 4MB would be the equivalent of what the current synchronous
> copy does now anyway?

I'm still a little worried about copy threads hanging indefinitely if
the peer goes away mid-copy.  The ability to signal the copy thread
would help.

--b.