Re: [PATCH] migration: Fix state transition in postcopy_start() error handling

[Peter Xu <peterx@redhat.com>]

On Sat, Sep 27, 2025 at 05:01:11PM +0300, Michael Tokarev wrote:
> On 26.08.2025 14:51, Juraj Marcin wrote:
> > From: Juraj Marcin <jmarcin@redhat.com>
> > 
> > Commit 48814111366b ("migration: Always set DEVICE state") introduced
> > DEVICE state to postcopy, which moved the actual state transition that
> > leads to POSTCOPY_ACTIVE.
> > 
> > However, the error handling part of the postcopy_start() function still
> > expects the state POSTCOPY_ACTIVE, but depending on where an error
> > happens, now the state can be either ACTIVE, DEVICE or CANCELLING, but
> > never POSTCOPY_ACTIVE, as this transition now happens just before a
> > successful return from the function.
> > 
> > Instead, accept any state except CANCELLING when transitioning to FAILED
> > state.
> > 
> > Cc: qemu-stable@nongnu.org
> > Fixes: 48814111366b ("migration: Always set DEVICE state")
> > Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
> > 
> > ---
> > In the RFC[1] where this patch was discussed, there was also a
> > suggestion for a helper function migrate_set_failure() that would check
> > if the state is not CANCELLING and then set migration error and FAILED
> > state. I discussed the implementation with Peter, and we came to a
> > conclusion that instead of patching such clean-up on top of the current
> > error handling code, it might be more useful to do a larger refactor and
> > clean-up of all error handling in the migration code.
> > 
> > Such clean-up should reduce the number of places where we need to
> > explicitly transition to a FAILED state (ideally to one, or only a
> > couple of places), and instead only set an appropriate migration error
> > using migrate_set_error(). Additionally, it would also refactor
> > inappropriate uses of QEMUFile errors where the error is not really an
> > error of the underlying channel and migrate_set_error() should be used
> > instead.
> > 
> > [1]: https://lore.kernel.org/all/20250807114922.1013286-3-jmarcin@redhat.com/
> 
> Ping?  Can we apply this to the master branch, so I can pick it up for
> the stable series?

Apologies for the delay, queued.  Will send the PR this week.

-- 
Peter Xu