* Yury Kotov (yury-ko...@yandex-team.ru) wrote:
> 18.04.2019, 19:03, "Dr. David Alan Gilbert" <dgilb...@redhat.com>:
> > * Yury Kotov (yury-ko...@yandex-team.ru) wrote:
> >> 18.04.2019, 17:20, "Dr. David Alan Gilbert" <dgilb...@redhat.com>:
> >> > * Yury Kotov (yury-ko...@yandex-team.ru) wrote:
> >> >> 15.04.2019, 14:30, "Dr. David Alan Gilbert" <dgilb...@redhat.com>:
> >> >> > * Daniel P. Berrangé (berra...@redhat.com) wrote:
> >> >> >> On Mon, Apr 15, 2019 at 12:15:12PM +0100, Dr. David Alan Gilbert
> >> wrote:
> >> >> >> > * Daniel P. Berrangé (berra...@redhat.com) wrote:
> >> >> >> > > On Mon, Apr 15, 2019 at 01:33:21PM +0300, Yury Kotov wrote:
> >> >> >> > > > 15.04.2019, 13:25, "Daniel P. Berrangé"
> >> <berra...@redhat.com>:
> >> >> >> > > > > On Mon, Apr 15, 2019 at 01:17:06PM +0300, Yury Kotov
> >> wrote:
> >> >> >> > > > >> 15.04.2019, 13:11, "Daniel P. Berrangé"
> >> <berra...@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.
> >> >> >> >
> >> >> >> > Hmm, that's true - although:
> >> >> >> > a) It's very non-obvious
> >> >> >> > b) Unfortunate, since it would go well with -incoming defer
> >> >> >>
> >> >> >> Yeah I think this is a screw up on QMEU's part when introducing
> >> 'defer'.
> >> >> >>
> >> >> >> We should have mandated use of 'add-fd' when using 'defer', since
> >> FD
> >> >> >> inheritance-over-execve() should only be used for command line
> >> args,
> >> >> >> not monitor commands.
> >> >> >>
> >> >> >> Not sure how to best fix this is QEMU though without breaking back
> >> >> >> compat for apps using 'defer' already.
> >> >> >
> >> >> > We could add mon-fd: transports that has the same behaviour as now
> >> for
> >> >> > outgoing, and for incoming uses the add-fd stash.
> >> >> >
> >> >>
> >> >> Oh, I'm sorry again. I think my suggestion about monitor_fd_param
> >> wasn't
> >> >> relevant to this issue. If migrate-incoming + "fd:" + add-fd is an
> >> invalid use
> >> >> case, should we disallow this?
> >> >> I may add a check to fd_start_incoming_migration if fd is in mon fds
> >> list.
> >> >> But I'm afraid there are users like me who are already using this
> >> wrong use case.
> >> >> Because currently nothing in QEMU's docs disallow this.
> >> >>
> >> >> So which solution is better in your opinion?
> >> >> 1) Disallow fd's from mon fds list in fd_start_incoming_migration
> >> >
> >> > I'm surprised anything could be doing that - how would a user know what
> >> > the correct fd index was?
> >> >
> >>
> >> Hmm, add-fd returns correct fd value. Maybe I din't catch you question...
> >
> > I don't understand, where does it return it?
> >
>
> From misc.json:
> # Example:
> #
> # -> { "execute": "add-fd", "arguments": { "fdset-id": 1 } }
> # <- { "return": { "fdset-id": 1, "fd": 3 } }
> #
>
> "fd": 3 is a valid fd for migrate-incoming(uri = "fd:3")
Ah OK.
> >> >> 2) Allow these fds, but dup them or close them correctly
> >> >
> >> > I think I'd leave the current (confusing) fd: as it is, maybe put a note
> >> > in the manual.
> >> >
> >>
> >> So, using fd from fdset will be an undefined behavior, right?
> >
> > For incoming, yes.
> >
> >> >> And how to migrate-incoming defer through fd correctly?
> >> >> 1) Add "mon-fd:" protocol to work with fds passed by
> >> "add-fd/remove-fd" commands
> >> >> as suggested by Dave
> >> >
> >> > That's my preference; it's explicitly named and consistent, and it
> >> > doesn't touch the existing fd code.
> >> >
> >>
> >> Ok, but please tell me what you think of my suggestion (2) about using fd
> >> added
> >> by the "getfd" command for incoming migration. It doesn't requires
> >> introducing
> >> new protocol and will be consistent with outgoing migration through fd.
> >
> > I worry how qemu knows whether the command means it comes from the getfd
> > command or is actually a normal fd like now?
> > Can you give an example.
> >
>
> getfd manages naming fds list.
> # -> { "execute": "getfd", "arguments": { "fdname": "fd1" } }
> So, for migrate (not incoming) is now valid migrate(uri="fd:fd1")
>
> I want the same for migrate-incoming. If fdname is parseable int, then it is
> an old format. Otherwise - it is a name of fd added by addfd.
>
> There is a function "monitor_fd_param" which do exactly what I mean:
> int monitor_fd_param(Monitor *mon, const char *fdname, Error **errp) {
> ... local vars ...
> if (!qemu_isdigit(fdname[0]) && mon) {
> fd = monitor_get_fd(mon, fdname, &local_err);
> } else {
> fd = qemu_parse_fd(fdname);
> }
> ... report err to errp ...
> }
OK, if we're already using monitor_fd_param everywhere then I think
we're already down the rat-hole of guessing whether we're an add-fd or
fd by whether it's an integer, and I agree with you that we should
just fix incoming to use that.
Now, that means I guess we need to modify monitor_fd_param to tell us
which type of fd it got, so we know whether to close it later?
Dave
P.S. I'm out from tomorrow for a weekish.
> >> >
> >> >> 2) My suggestion about monitor_fd_param and make "fd:" for
> >> >> migrate/migrate-incoming consistent. So user will be able to use
> >> >> getfd + migrate-incoming
> >> >> 3) Both of them or something else
> >> >>
> >>
>
> Regards,
> Yury
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
