Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Apr 15, 2019 at 01:33:21PM +0300, Yury Kotov wrote:
> 15.04.2019, 13:25, "Daniel P. Berrangé" <berrange@redhat.com>:
> > On Mon, Apr 15, 2019 at 01:17:06PM +0300, Yury Kotov wrote:
> >>  15.04.2019, 13:11, "Daniel P. Berrangé" <berrange@redhat.com>:
> >>  > On Mon, Apr 15, 2019 at 12:50:08PM +0300, Yury Kotov wrote:
> >>  >>  Hi,
> >>  >>
> >>  >>  Just to clarify. I see two possible solutions:
> >>  >>
> >>  >>  1) Since the migration code doesn't receive fd, it isn't responsible for
> >>  >>  closing it. So, it may be better to use migrate_fd_param for both
> >>  >>  incoming/outgoing and add dupping for migrate_fd_param. Thus, clients must
> >>  >>  close the fd themselves. But existing clients will have a leak.
> >>  >
> >>  > We can't break existing clients in this way as they are correctly
> >>  > using the monitor with its current semantics.
> >>  >
> >>  >>  2) If we don't duplicate fd, then at least we should remove fd from
> >>  >>  the corresponding list. Therefore, the solution is to fix qemu_close to find
> >>  >>  the list and remove fd from it. But qemu_close is currently consistent with
> >>  >>  qemu_open (which opens/dups fd), so adding additional logic might not be
> >>  >>  a very good idea.
> >>  >
> >>  > qemu_close is not appropriate place to deal with something speciifc
> >>  > to the montor.
> >>  >
> >>  >>  I don't see any other solution, but I might miss something.
> >>  >>  What do you think?
> >>  >
> >>  > All callers of monitor_get_fd() will close() the FD they get back.
> >>  > Thus monitor_get_fd() should remove it from the list when it returns
> >>  > it, and we should add API docs to monitor_get_fd() to explain this.
> >>  >
> >>  Ok, it sounds reasonable. But monitor_get_fd is only about outgoing migration.
> >>  But what about the incoming migration? It doesn't use monitor_get_fd but just
> >>  converts input string to int and use it as fd.
> >
> > The incoming migration expects the FD to be passed into QEMU by the mgmt
> > app when it is exec'ing the QEMU binary. It doesn't interact with the
> > monitor at all AFAIR.
> >
> 
> Oh, sorry. This use case is not obvious. We used add-fd to pass fd for
> migrate-incoming and such way has described problems.

That's a bug in your usage of QEMU IMHO, as the incoming code is not
designed to use add-fd.

Regards,
Daniel
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