Ping
18.04.2019, 20:46, "Yury Kotov" <yury-ko...@yandex-team.ru>:
> 18.04.2019, 20:01, "Dr. David Alan Gilbert" <dgilb...@redhat.com>:
>> * 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.
>
> I think the right way is to check whether fd is added by add-fd and if so then
> return error. Because IIUC the only valid usage of add-fd is to use
> qemu_open("/dev/fdset/<fdset_id>") which finds suitable fd from fdset.
> Such behavior is incompatible with fd:<fd_num> at all, as such syntax
> doesn't imply the using of particular fd. But if so, why add-fd returns
> the value of added fd?..
>
> But if I'm right it's enough to:
> 1) Modify monitor_fd_param to check where fd comes from and disallow using
> fd of "add-fd",
> 2) As we discussed earlier, modify monitor_get_fd to remove named fd from its
> list before return,
Omg, monitor_fd_param is already do so... Sorry, so the problem only in
incoming case.
> 3) Use monitor_fd_param in migrate_incoming for "fd:" proto.
>
> I'm not insist. May be it's ok to use fd from fdset directly and so qemu_close
> should be modifyed.
>
> Just to clarify what I mean:
> fdset is a struct:
> struct MonFdset {
> int64_t id;
> QLIST_HEAD(, MonFdsetFd) fds;
> QLIST_HEAD(, MonFdsetFd) dup_fds;
> QLIST_ENTRY(MonFdset) next;
> };
>
> * add-fd appends new fd to "->fds" list.
> * qemu_open("/dev/fdset/X", int perms) is looking for suitable (by perms) fd
> from fdset with id X, dup it and append "->dup_fds" list.
> * qemu_close(int fd) tryes to find the fd in all "->dup_fds" lists
> of all fdsets and remove it. And closes fd anyway.
>
> If not to disallow using fds added by add-fd then there are three
> possible solutions:
> 1) dup fd in monitor_fd_param it the fd is from some fdset,
> 2) remove the fd from "->fds" list in qemu_close
> 3) don't close it in qemu_close, so client is responsible to close it by
> remove-fd.
>
>>> >> >
>>> >> >> 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
