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

[Yury Kotov]

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.
>

May be it's better to use monitor_fd_param for both incoming/outgoing?
So, "migrate" will know fd:<int> semantics and "migrate-incoming" will
know fd:<fd_name> semantics. And also modify monitor_get_fd to
remove fd from list before return.
This is a backwards compatible change.

Regards,
Yury