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Re: [libvirt] [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd





On 07/09/2012 11:05 AM, Corey Bryant wrote:


On 07/09/2012 10:05 AM, Luiz Capitulino wrote:
On Thu, 05 Jul 2012 11:06:56 -0400
Corey Bryant <coreyb linux vnet ibm com> wrote:



On 07/04/2012 04:09 AM, Kevin Wolf wrote:
Am 03.07.2012 20:21, schrieb Corey Bryant:
On 07/03/2012 02:00 PM, Eric Blake wrote:
On 07/03/2012 11:46 AM, Corey Bryant wrote:


Yes, I think adding a +1 to the refcount for the monitor makes
sense.

I'm a bit unsure how to increment the refcount when a monitor
reconnects
though.  Maybe it is as simple as adding a +1 to each fd's
refcount when
the next QMP monitor connects.

Or maybe delay a +1 until after a 'query-fds' - it is not until the
monitor has reconnected and learned what fds it should be aware of
that
incrementing the refcount again makes sense.  But that would mean
making
'query-fds' track whether this is the first call since the monitor
reconnected, as it shouldn't normally increase refcounts.

This doesn't sound ideal.

Yes, it's less than ideal.

The other alternative is that the monitor never re-increments a
refcount.  Once a monitor disconnects, that fd is lost to the
monitor,
and a reconnected monitor must pass in a new fd to be
re-associated with
the fdset.  In other words, the monitor's use of an fd is a one-way
operation, starting life in use but ending at the first disconnect or
remove-fd.

I would vote for this 2nd alternative.  As long as we're not
introducing
an fd leak.  And I don't think we are if we decrement the refcount on
remove-fd or on QMP disconnect.

In fact, I believe this one is even worse. I can already see hacks like
adding a dummy FD with invalid flags and removing it again just to
regain control over the fdset...

You earlier suggestion made a lot of sense to me: Whenever a new QMP
monitor is connected, increase the refcount. That is, as long as any
monitor is there, don't drop any fdsets unless explicitly requested
via QMP.

Ok.  So refcount would be incremented (for the fd or fdset, whatever we
decide on) when QMP reconnects.  I'm assuming we wouldn't wait until
after a query-fds call.

I'm not sure this is a good idea because we will leak fds if the
client forgets
about the fds when re-connecting (ie. it was restarted) or if a different
client connects to QMP.

If we really want to do that, I think that the right way of doing this
is to
add a command for clients to re-again ownership of the fds on
reconnection.

But to be honest, I don't fully understand why this is needed.


I'm not sure this is an issue with current design.  I know things have
changed a bit as the email threads evolved, so I'll paste the current
design that I am working from.  Please let me know if you still see any
issues.

FD passing:
-----------
New monitor commands enable adding/removing an fd to/from a set.  New
monitor command query-fdsets enables querying of current monitor fdsets.
  The set of fds should all refer to the same file, with each fd having
different access flags (ie. O_RDWR, O_RDONLY).  qemu_open can then dup
the fd that has the matching access mode flags.

Design points:
--------------
1. add-fd
-> fd is passed via SCM rights and qemu adds fd to first unused fdset
(e.g. /dev/fdset/1)
-> add-fd monitor function initializes the monitor inuse flag for the
fdset to true
-> add-fd monitor function initializes the remove flag for the fd to false
-> add-fd returns fdset number and received fd number (e.g fd=3) to caller

2. drive_add file=/dev/fdset/1
-> qemu_open uses the first fd in fdset1 that has access flags matching
the qemu_open action flags and has remove flag set to false
-> qemu_open increments refcount for the fdset
-> Need to make sure that if a command like 'device-add' fails that
refcount is not incremented

3. add-fd fdset=1
-> fd is passed via SCM rights
-> add-fd monitor function adds the received fd to the specified fdset
(or fails if fdset doesn't exist)
-> add-fd monitor function initializes the remove flag for the fd to false
-> add-fd returns fdset number and received fd number (e.g fd=4) to caller

4. block-commit
-> qemu_open performs "reopen" by using the first fd from the fdset that
has access flags matching the qemu_open action flags and has remove flag
set to false
-> qemu_open increments refcount for the fdset
-> Need to make sure that if a command like 'block-commit' fails that
refcount is not incremented

5. remove-fd fdset=1 fd=4
-> remove-fd monitor function fails if fdset doesn't exist
-> remove-fd monitor function turns on remove flag for fd=4

6. qemu_close (need to replace all close calls in block layer with
qemu_close)
-> qemu_close decrements refcount for fdset
-> qemu_close closes all fds that have (refcount == 0 && (!inuse ||
remove))
-> qemu_close frees the fdset if no fds remain in it

7. disconnecting the QMP monitor
-> monitor disconnect visits all fdsets on monitor and turns off monitor
in-use flag for fdset

8. connecting the QMP monitor
-> monitor connect visits all fdsets on monitor and turns on monitor
in-use flag for fdset

9. query-fdsets
-> returns all fdsets and fds that don't have remove flag on

QMP command examples
--------------------
-> { "execute": "add-fd", "arguments": { "fdset": 1 } }
<- { "return": { "fdset": 1, "fd": 3 } }

-> { "execute": "remove-fd", "arguments": { "fdset": 1, "fd": 3 } }
<- { "return": {} }

-> { "execute":"query-fdsets" } =>
<- { "return" : { "fdsets": [
      { "name": "fdset1",
        "fds": [ { "fd": 4, "removed": false } ],
        "refcount": 1,
        "monitor": true },
      { "name": "fdset2",
        "fds": [ { "fd": 5, "removed": false },
                 { "fd": 6, "removed": true } ],
        "refcount": 1,
        "monitor": true } }


Here's a minor correction to query-fdsets. s/name/fdset and I changed fdset to an int to stay in sync with add-fd and remove-fd.

-> { "execute":"query-fdsets" } =>
<- { "return" : { "fdsets": [
     { "fdset": 1,
       "fds": [ { "fd": 4, "removed": false } ],
       "refcount": 1,
       "monitor": true },
     { "fdset": 2,
       "fds": [ { "fd": 5, "removed": false },
                { "fd": 6, "removed": true } ],
       "refcount": 1,
       "monitor": true } }

--
Regards,
Corey



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