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Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol |
Date: |
Thu, 18 Apr 2019 15:19:56 +0100 |
User-agent: |
Mutt/1.11.4 (2019-03-13) |
* Yury Kotov (address@hidden) wrote:
> 15.04.2019, 14:30, "Dr. David Alan Gilbert" <address@hidden>:
> > * Daniel P. Berrangé (address@hidden) wrote:
> >> On Mon, Apr 15, 2019 at 12:15:12PM +0100, Dr. David Alan Gilbert wrote:
> >> > * Daniel P. Berrangé (address@hidden) wrote:
> >> > > On Mon, Apr 15, 2019 at 01:33:21PM +0300, Yury Kotov wrote:
> >> > > > 15.04.2019, 13:25, "Daniel P. Berrangé" <address@hidden>:
> >> > > > > On Mon, Apr 15, 2019 at 01:17:06PM +0300, Yury Kotov wrote:
> >> > > > >> 15.04.2019, 13:11, "Daniel P. Berrangé" <address@hidden>:
> >> > > > >> > 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?
> 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.
> 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.
Dave
> 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 Kotov
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, (continued)
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Daniel P . Berrangé, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Daniel P . Berrangé, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Dr. David Alan Gilbert, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Daniel P . Berrangé, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Dr. David Alan Gilbert, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/15
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/16
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/16
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol,
Dr. David Alan Gilbert <=
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/18
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Dr. David Alan Gilbert, 2019/04/18
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/18
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Dr. David Alan Gilbert, 2019/04/18
- Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol, Yury Kotov, 2019/04/18