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

[Yury Kotov]

18.04.2019, 19:03, "Dr. David Alan Gilbert" <address@hidden>:
> * Yury Kotov (address@hidden) wrote:
>>  18.04.2019, 17:20, "Dr. David Alan Gilbert" <address@hidden>:
>>  > * 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?
>>  >
>>
>>  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")

>>  >>  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 ...
}

>>  >
>>  >>  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