migration/postcopy: Fix high frequency sync

migration/postcopy: Fix high frequency sync

With current code base I can observe extremely high sync count during
precopy, as long as one enables postcopy-ram=on before switchover to
postcopy.

To provide some context of when QEMU decides to do a full sync: it checks
must_precopy (which implies "data must be sent during precopy phase"), and
as long as it is lower than the threshold size we calculated (out of
bandwidth and expected downtime) QEMU will kick off the slow/exact sync.

However, when postcopy is enabled (even if still during precopy phase), RAM
only reports all pages as can_postcopy, and report must_precopy==0.  Then
"must_precopy <= threshold_size" mostly always triggers and enforces a slow
sync for every call to migration_iteration_run() when postcopy is enabled
even if not used.  That is insane.

It turns out it was a regress bug introduced in the previous refactoring in
8.0 as reported by Nina [1]:

  (a) c8df4a7aef ("migration: Split save_live_pending() into state_pending_*")

Then a workaround patch is applied at the end of release (8.0-rc4) to fix it:

  (b) 28ef5339c3 ("migration: fix ram_state_pending_exact()")

However that "workaround" was overlooked when during the cleanup in this
9.0 release in this commit..

  (c) b0504edd40 ("migration: Drop unnecessary check in ram's pending_exact()")

Then the issue was re-exposed as reported by Nina [1].

The problem with (b) is that it only fixed the case for RAM, rather than
all the rest of iterators.  Here a slow sync should only be required if all
dirty data (precopy+postcopy) is less than the threshold_size that QEMU
calculated.  It is even debatable whether a sync is needed when switched to
postcopy.  Currently ram_state_pending_exact() will be mostly noop if
switched to postcopy, and that logic seems to apply too for all the rest of
iterators, as sync dirty bitmap during a postcopy doesn't make much sense.
However let's leave such change for later, as we're in rc phase.

So rather than reusing commit (b), this patch provides the complete fix for
all iterators.  When at it, cleanup a little bit on the lines around.

[1] https://gitlab.com/qemu-project/qemu/-/issues/1565

Reported-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Fixes: b0504edd40 ("migration: Drop unnecessary check in ram's pending_exact()")
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240320214453.584374-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>